Chelsea’s improved defence by the numbers: A statistical analysis of the centre back options

The key tenet to Thomas Tuchel’s approach as Chelsea manager was to immediately and successfully employ a three at the back defensive system, three centre back choices (CBs) were used alongside two wingbacks in a similar manner to Antonio Conte in 2016-2018. The effects were devastating for opponent’s attacking play and the Blues hardly conceded goals since late January after Tuchel inked pen to paper on his contract to become the manager.

It was clear that each centre back massively improved in the German manager’s formation and system, but just how good was each over the course of the season, and are there any CBs who stood out more than others at some aspects of the game? To answer this, a statistical analysis of the end-of-season data was performed and reported below.

A paramount responsibility of any centre back is defensive contributions in the form of breaking up play, marking attackers, pressuring the opponent, and knowing when and where passes will be played. Within the back four that was used prior to Tuchel, many of the back line struggled in defensive responsibilities, but after Tuchel arrived seemed to form a highly cohesive, resolute unit that was mostly impenetrable. Despite the eye test telling the story that all of the CBs had a fairly positive year, what does the full season data say?

Author’s note: All data analyzed, unless otherwise stated, is the data from the entire 2020/2021 Premier League Season.

How high do the centre back options engage?

Cesar Azpilicueta led all the centre back choices at Chelsea this past season in defensive, middle and attacking third tackles. Thiago Silva and Antonio Rudiger were quite a margin behind Azpilicueta in defensive and middle third tackles, but Rudiger was second in the squad for middle third tackles and tied with Silva for second in defensive third tackles. Some of this can be explained by Azpilicueta playing at fullback and wingback several times throughout the season, particularly prior to the arrival of the rising German mastermind, Thomas Tuchel.

A chart to show how the Chelsea centre back choices engage up the field per third.
A chart to show how the Chelsea centre back choices engage up the field per third.

The above graph shows a few things. The number of minutes is important to consider. Azpilicueta had more minutes (2,188 minutes) than any other centre back analyzed. Despite having more minutes, he disproportionately produced more tackles in all thirds of the field compared to Silva (1,935 minutes) and Zouma (2,029 minutes). Only Rudiger (1,711 minutes) and Andreas Christensen (1,371 minutes) would have a claim to have equal defensive and middle third tackles if given the same minutes as Azpilicueta.

As a captain, this is a key stat as he is showing the positioning and movement to take on opposing attackers and win the ball from them as they are moving towards him. It also shows that Azpilicueta pushes higher up the pitch to win the ball with more success than any other centre back. However, this also shows that Chelsea have two other CBs in Rudiger and Christensen who are likely able to produce the same output as Azpilicueta if given sufficient minutes next season, especially if time and age begin to become a factor for Captain Azpilicueta.

Do Chelsea need to teach tackling?

“The second balls is a concept that is typical here in England when they use a lot about the tackles. I am not a coach for the tackles so I don’t train the tackles. What I want is to try and play good and score goals and arrive more [in the box]. 

“Yes the duels you have to win that’s true. But normally when you play good you win a lot of tackles but after four minutes 2-0 in the mind of the players [they say] ‘what’s going on, what happened’ and it’s not easy for them. That’s why it’s another aspect of the football but in the end we’re not going to win or lose for the tackles.” 

Pep Guardiola speaking after his Manchester City side lost to Leicester City in the Spaniards first season in England

Tackles are far from the only stat that matters for a defender. Defensive actions such as interceptions, pressures, and clearances are also informative. Azpilicueta produced the most interceptions this past season, but had Christensen played an equal number of minutes to the Spanish veteran, it is reasonable to assume Christensen would have been leading this category. This shows that the Dane is a supreme reader of the opposition’s attacking movements, and that he has good positioning and anticipation of the ball. This also shows the same for Azpilicueta, but Christensen’s stats are more impressive considering he played almost half as many minutes. Silva continues to show that with age does not come the inability to read the game, as he was second in interceptions.

A chart to show the number of clearances and interceptions by Chelsea's centre back options
A chart to show the number of clearances and interceptions by Chelsea’s defenders

Zouma had considerably more clearances than all but Silva, and Christensen had the lowest overall with Rudiger having a slightly higher number than the Danish international. Interpreting this is difficult but could be one of three things.

First, this could mean that Kurt Zouma is best equipped at being the final stopgap in defense who gets into positions to mop up the panic moments in defense when backs are against the wall in the defensive third. Conversely, this could also suggest Zouma has the best recovery pace of any centre back and is able to track down final runs and clear the ball before more danger is enacted on the goalkeeper.

Second, this could also mean that Zouma gets into scenarios where he is not able to play a pass to get out of pressure in his defensive third, and instead clears the ball up the pitch to let the other squad reset and re-establish their defensive shape. Finally, this could mean an interaction of the two factors just described. Given the eye test, I would shade towards the first assertion being the most truthful, but still must give some credit to the idea there is an interaction of effects.

A defender can also use pressures to indirectly stop the opponent from successful attacking play, instead of relying on tackling or interceptions to directly break up the opponent’s play. Azpilicueta produced the most pressures in all thirds of the pitch last season. This, on top of his squad leading number of tackles, shows that he is highly effective at directly and indirectly disrupting the opponent.

He is a complete defender, and this is evidence by him being deployed at multiple positions along the defensive front across Lampard and Tuchel. However, had Rudiger been given nearly equal minutes to the captain, it is likely he would have been close to Azpilicueta in defensive third pressures. The versatile defender also has a very healthy margin of victory for middle third pressures, which shows he is able to go off his defensive line and be proactive at disrupting play.

Finally, Azpilicueta had significantly more attacking third pressures. El Capitan wins this category by quite some margin, which highlights how effective he is at pressuring up and down the pitch to indirectly stop the opponent or disrupt their passing movements.

A graph detailing the pressures put on by Chelsea centre back options last season.
A graph detailing the pressures put on by Chelsea centre back options last season.

Another aspect of tackling is how the tackle is made: is it against a dribbler, is it against a shot attempt, is it against a 50/50 ball from a slightly misplaced pass?

The ability to drop a tackle and take the ball from an opponent dribbling directly at a defender is crucial. With respect to tackles against dribble attempts, Rudiger led all centre backs with an impressive 66.7% success rate of tackles against dribbles. Two out of every three dribbles that Rudiger attempted a tackle, he successfully took the ball from the opponent. This is a highly impressive stat and underpins just how good he is at ground duels.

A graph to show how Chelsea's centre backs tackle per dribble attempt.
A graph to show how Chelsea’s centre backs tackle per dribble attempt.

However, the ever-present Azpilicueta was not far behind with 60.0% of tackles won against dribbles. Zouma, is who usually thought of as an excellent tackler of the ball, was third in this category with 54.5% success rate for tackles against dribbles.

The last bit of defending a centre back could have to do is to block a pass, cross, or shot attempt. This statistic revealed that Azpilicueta produced the most overall blocks, but that many of Azpilicueta’s blocks were not from shots.

A chart to explain the number of blocks from Chelsea defensive options.
A chart to explain the number of blocks from Chelsea defensive options.

Interestingly, Christensen produced 15 blocked shots and 26 other blocks, which highlights that he is doing very well at his positioning and anticipation, while also closing down space quickly, relative to the attacker to block the shot attempt as it is happening. Also, nearly half of Rudiger’s blocks were blocked shots.

This graph highlights that Rudiger and Christensen were the best at blocking shots, while the Dane and Azpilicueta were overall the best at blocking the ball in general. However, had Rudiger and Christensen been given equal minutes to Azpilicueta, they would each be the leaders in this category.

Overall, there is reason to believe from the above data that Rudiger and Christensen will continue to grow and continue their effective talents at stopping shots from opponents.

Tuchel’s centre back passing abilities examined

The ability of a defender to sit deep and play passes as become more of a key role in recent years. Watching Chelsea this season showed plenty of times how effective that Silva was at this skill. Many times it can be remembered that Silva was breaking the opponent’s defensive lines with balls over the top to attackers. What does the data show for passing?

How Chelsea's centre backs play from the defence, examined in this chart.
How Chelsea’s centre backs play from the defence, examined in this chart.

From the graph, it is evident that the pass completion percentages for short and medium passes is fairly similar for all the players analysed. However, Rudiger and Silva were the first and second CBs in short pass completion percentage, at 96.5 and 96.3 percent, respectively. Silva had the best medium pass completion percentage at 97.1%, and Rudiger was second with 96.1 completion percentage for medium passes. Finally, Christensen had the best long pass completion percentage (86.3%) and Silva was second at 82.3%.

Overall, this shows that Rudiger and Christensen are well equipped to fill in for the strengths that Silva offers when Silva is not available or being rested. Also, the passing statistics from all of the CBs are quite impressive. Specifically, Rudiger, Silva, and Christensen are standouts in each category and perhaps can be trusted more with taking up roles that will sometimes use them to break the lines of the opponent.

Additionally, the types of passes and was also analysed to see if any centre back is providing more key passes or passes into the final third when compared to their pass completion percentages above.

ADD BELOW IN IMAGE FIELD GRAPH TITLED ATTACKINGTHIRDPASSES AND DELETE THIS LINE WHEN FINISHED EDITING

A graph to show the attacking passes played by Chelsea's centre back players.
A graph to show the attacking passes played by Chelsea’s centre back players.

Azpilicueta had the most progressive passes compared to all others, but again, he did play at times at fullback and wingback which could be contributing to this. Silva showed that his ability to complete long passes is also in a manner that moves the ball more directly towards the opponents goal (i.e. he does not play long passes that do not travel 10+ yards towards the opponent’s goal). Also, Silva played entirely as a centre back this past season, and marginally trails Azpilicueta for the most passes completed into the final third, with Rudiger very close to Silva. However, Azpilicueta played significantly more key passes than any other centre back.

Where the goals are conceded

Finally, we examine the median goals that were conceded under Tuchel per each centre. We also explored the variance around the medium and the percentiles above and below the median to get a better sense of how each centre back is likely to underperform or overperform the expected median average. A boxplot was made for this. This data is only analyzing the matches that Tuchel was in charge.

A plot to examine where the goals have been conceded since Thomas Tuchel took over at Chelsea.
A plot to examine where the goals have been conceded since Thomas Tuchel took over at Chelsea.

The plot above shows a few things: the bold line is the median, the lower bound of the grey box is the 25th interquartile range, and the upper bound of the grey box is the 75th interquartile range. The hollow dots above the plot are data points that were outliers.

What can be immediately gathered from this chart is that the median goals allowed for each centre back is zero, except for Silva. The reason for the median being zero goals allowed for four of the five CBs is because Chelsea, under Tuchel, rarely conceded goals and each player had many clean sheets.

However, the boxplot does show that Silva did concede more goals than the others, per the median, and that his 75th IQR is the same as the median. Overall, this is suggestive there was little variation in the number of goals conceded, which is explained by the previous analysis that Tuchel’s team hardly conceded goals over four months.

What is most interesting, however, is that Christensen’s median is zero, and even when he allowed a goal, it was a statistical outlier per his sample of games and goals allowed in each match. This is ridiculously impressive, and combined with all above graphs, shows that the Dane has really had a bounce back season and proved his worth, along with Rudiger.

For a bit of comparative fun, the number of goals per centre back was re-analysed, but this time only using data from Lampard’s time as manager for the 2020/2021 season in a boxplot and displayed below.

A graph to examine the goals conceded by each Chelsea centre back under Frank Lampard this season.
A graph to examine the goals conceded by each Chelsea centre back under Frank Lampard this season.

It is immediately apparent that all CBs had more variance around the median, and that all but Azpilicueta and Silva had a higher median of goals conceded. Specifically, Christensen was significantly worse and much more variable without Tuchel.

Interestingly, Ruidger had the lowest median goals conceded yet played the least number of times in 2020/2021 under Lampard. Silva still had a median goal conceded of 1.0 but he had a massively increased amount of variance around the median, which underpins that results were very mixed with Silva and that he couldn’t string together consistency in the back four.

Overall, the move to three at the back was a revelation, and the above two boxplots under Tuchel and Lampard, respectively, highlight just how effective it was and in large part reveal that the formation switch was integral and paramount to the Blues lifting the Champions League.

Written by Travis Flock (@Crossroads_CFC)

Edited by Tom Coley (@tomcoley49)

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