Saturday, 1 January 2011

Mid-season analysis: What's wrong with Arsenal's central defence?

We're exactly half way through the Premier League season and after another 2 cheap points were dropped at Wigan recently, it's worth exploring what's going wrong at the heart of Arsenal's defence, especially when it's occupied by the French pair of Laurent Koscielny and Sebastian Squillaci. The goalkeeper and left back situations have been scrutinised in great detail by Arsenal fans but the purpose of this article will be to examine the centre backs alone through a combination of statistical and video analysis.

For the sake of a fair comparison, Champions League and League Cup games will be ignored due to the weakened nature of the teams fielded. Tackles and interceptions made by centre backs in the final 3rd of the pitch will be excluded from the stats as these generally only occur during corner kick situations and do not reflect defensive ability. The 3rd goal conceded against West Brom in the 2-3 home defeat will also be ignored as the team was playing a 3-man defence from the 66th minute onwards (tackles/interceptions/shots during that time will also be ignored).


Firstly, let's take a look at the defensive efficiency of the 5 different centre backs pairings that have been fielded during the Premier League campaign so far:

Table 1: Centre back statistics for all 19 Premier League games to date

The Djourou-Koscielny and Song-Vermaelen partnerships have had little game time so can be ignored from this comparison. With that in mind the key points are that:

-The Koscielny-Squillaci pairing is the leakiest and concedes goals at almost 4 times the rate of the most secure pairing, Djourou-Squillaci.
-Despite conceding the least goals, the Djourou-Squillaci partnership actually concedes the most total shots and the 2nd most shots on target.
-The Koscielny-Squillaci partnership concedes fewer total shots than any other partnership, but the most shots on target.
-The Koscielny-Vermaelen pairing concedes the fewest total shots and the fewest shots on target.

These stats are for all Premier League games and so do not take into account the differences between home games and away games (where defences generally come under extra attacking pressure). Therefore here is a further breakdown of these stats beginning with home games:

Table 2: Centre back statistics for all 9 Premier League home games to date

From this table the only 2 partnerships worth comparing are Djourou-Squillaci and Koscielny-Squillaci. Again, key points are that:

-The Djourou-Squillaci pairing is the most secure and concedes goals at almost 3 times a lower rate than Koscielny-Squillaci.
-The Djourou-Squillaci partnership concedes more total shots and more shots on target than Koscielny-Squillaci.

And finally the away performances:

Table 3: Centre back statistics for all 10 Premier League away games to date

Key points here are that:

-Djourou-Squillaci once again emerges as the most secure pairing, despite conceding the most total shots.
-Koscielny-Squillaci is the least secure and concedes the most shots on target (as opposed to the least in home performances).
-Koscielny-Vermaelen concedes the least total shots and the least shots on target.

With the exception of shots on target conceded by Koscielny-Squillaci, the pattern of home and away games is quite similar.


So what is the reason behind these trends? To understand we need to take a closer look at the strengths and weaknesses of each individual centre back, so that we can recognise the dynamics of each separate pairing. Here are individual stats for interceptions, aerial duels and tackling:

Table 4: Individual defensive stats for the 4 centre backs

The points we can draw from this are that:

-Koscielny is the most pro-active defender, attempting the most tackles and aerial duels, and the 2nd most interceptions. His tackling is by far the tidiest.
-Squillaci prefers to keep his position rather than close down, attempting fewer interceptions, aerial duels and tackles.
-Vermaelen and Djourou are somewhere in between these 2 varying styles, Vermaelen tending more towards Koscielny's game while Djourou is more akin to Squillaci.
-Despite attempting the most aerial duels, Koscielny is joint weakest in this area along with Squillaci (who attempts by far the fewest).
-Vermaelen is the strongest in the air, with Djourou not far behind.


With this knowledge, we can roughly rank the centre backs in 2 important areas of the game:

Closing down:
Koscielny = Vermaelen > Djourou = Squillaci

Aerial ability:
VermaelenDjourou > Koscielny = Squillaci

This helps us to better understand the dynamics of each centre back pairing:

-Koscielny-Vermaelen concedes the fewest shots on target because it closes down the best and thwarts attacks before they develop (with a larger sample size it could possibly rank the lowest in terms of total shots too).
-The Djourou-Squillaci partnership concedes the most total shots because it doesn't close down (it is joint worst for shots on target too).
-The Koscielny-Squillaci pairing is the leakiest because it is weak in the air and therefore vulnerable from aerial balls and set pieces.
-The Djourou-Squillaci pairing is the strongest because it is deep-lying and doesn't risk being caught high up the pitch. Instead it looks to soak up opposition pressure, taking advantage of Djourou's aerial ability.

The major anomaly within these stats is that the Koscielny-Squillaci partnership concedes few total shots (due mainly to Koscielny's closing down?) but concedes the most shots on target and indeed the most goals (by a large distance). If the partnership restricts opposition shots so much, why then do a high proportion of these end up on target and in the net?

Comparing the Squillaci-Djourou and Squillaci-Koscielny partnerships, it's difficult to comprehend that the vast difference in defensive efficiency is down to a simple difference in defensive strategy. The fact that replacing Johan Djourou with Laurent Koscielny results in an almost 4-fold increase in goals being conceded, should be cause for concern, and points the finger of blame at the young Frenchman.

So what exactly is Koscielny doing wrong? It could be that he struggles on the left side of defence when partnered with Squillaci, as opposed to the more natural right-sided role he assumes when partnered with Vermaelen. Another factor may be that without the comfort of a Premier League-experienced partner, Koscielny lacks the right support and leadership to effectively function within this defence.

Another more simple explanation would be that Laurent Koscielny is simply error-prone, and that despite all his good work in getting tight to opponents and winning the ball back early, he lets it all go to waste with one or two moments of madness each match, whether it's a positional or technical error.

As usual, this pattern is best demonstrated with actual graphic evidence from the matches. The following video breaks down the 21 goals conceded in the Premier League campaign so far (excluding the 3rd against West Brom), and we can observe that indeed the main problems of the Squillaci-Koscielny partnership are its aerial weakness and Koscielny's tendency to make costly mistakes:


So what can be done to remedy the current situation? The obvious decision would be to drop Koscielny in the short term and persist with the Squillaci-Djourou partnership, certainly in away games anyway. At home Koscielny's style of play would be beneficial towards Arsenal's pressing game and he can compete with Squillaci to partner the excellent Djourou (and when fit, Vermaelen).

In the long term, with more Premier League and general defensive experience, Koscielny should mature into a fine modern centre back, combining his excellent technical qualities and intelligence with improved concentration and reliability. It will be of great interest to see which two of Djourou, Koscielny and Vermaelen will establish themselves as the club's first choice centre backs given their similar ages and differing qualities.