Arsenal have conceded 41 goals over 30 League matches this season for a total of 1.41 goals allowed per game. That's Arsene Wenger's worst defensive return in his 21 seasons at Arsenal.
Arsenal's 2nd worst defensive season under Wenger was 2011/12 - the infamous "Trolly Dash" season, when Arsenal conceded 14 goals in the first 5 matches. Arsenal went on to conceded 49 goals that season, 1.29 per game. But after Mertesacker and Arteta settled in, Arsenal returned to a much more normal pace, conceding 35 goals in 33 matches which is 1.06 goals allowed per game, almost exactly Wenger's career average at Arsenal of 0.98 goals per game.
I mention 2011/12 only to illustrate how awful this season has been for Arsenal's defense. It's worse than the season in which Arsenal conceded 8 goals in a single match.
Those of you who know me know that I keep a database of shots taken and faced by Arsenal. Not all shots are the same value: a shot taken from outside the 18 yard box is scored on average 3% of the time; a shot inside the 18 yard box about 8% of the time, and a shot in the prime areas (slight angles in front of goal up to about 15 yards) are scored at about a 10% rate. There are a few other special types of shots. A penalty is the most obvious but there are times when a player gets such a great shot that fans think "oh he has to score that". These are called "Big Chances" by Opta and they are the single most important type of shot that a player can get. You'll often hear a manager say "we have to take our chances" these are the shots that they are talking about.
Let's look at the League stats for Big Chances this season. This season there have been 784 goals scored and 456 of them have been Big Chances. 58% of this season’s goals have been Big Chances. Big Chances are the most important shots stat collected right now.
Arsenal are conceding fewer shots per game (10.67) than they have over the last four years (11.5 average) and that would seem good but they are conceding more Big Chances than they have since I started collecting this data three years ago. Arsenal are now 8th worst in the Premier League for Big Chances allowed:
One might be tempted to blame Cech and Cech's Big Chance saves numbers are also down from his last three years:
But, while Cech's Big Chance save % this season looks awful but it's actually just average. A lot of attention has been given to David de Gea's saves numbers this season and rightly so, he's saved 45% of the big chances he's faced this season, 3rd best in the League. The League average this season is 35% and even top clubs like Liverpool and Tottenham are saving 31% or below. So, while Cech is saving fewer Big Chances than he did when he first came to Arsenal, he's just reverted to the mean.
The bigger problem is that Arsenal are conceding so many Big Chances.
Man City have the best defense in the League in Big Chances allowed with just 24 (they have the best defense in the League full stop). But Tottenham are second with 30, Chelsea are 3rd with 31, Liverpool are 4th with 41, and even Man U have allowed only 44 Big Chances this season. Arsenal's 50, plus 5 penalties, is really awful.
Where Cech has been at fault, however, is in errors. Cech himself has allowed 7 shots off errors this season and the opposition has scored 6 of them as goals. Every one of those shots were Big Chances. That means Cech is responsible for 14% of Arsenal's Big Chances allowed.
If you remove Cech's errors, Arsenal are still allowing a three year high of 1.43 Big Chances per game but Cech's Big Chance save percent goes up to 39%. Which is very respectable.
So, while Cech is saving at a normal human rate, again, the problem with Arsenal’s defense is that they are simply conceding too many Big Chances.
Arsenal have conceded 55 Big Chances this season, 8th worst in the League - sandwiched between Swansea (56) and Huddersfield (55).
Removing penalties, Arsenal are still 8th worst in the League in Big Chances allowed with 50, sandwiched between Swansea (52) and Huddersfield (49).
Arsenal’s overall defensive problems are exacerbated by Cech’s 7 errors, 6 leading to a goal (the worst in the Premier League this season). And if we remove Cech’s errors, Arsenal’s Big Chances allowed as a team drop to 43. That’s better, but still 10th worst in the League, sandwiched between West Brom (45) and West Ham (43).
That means Arsenal’s team defense is 10th worst in the League and Arsenal are leading the League with 28 errors and 13 goals off errors. Meanwhile, Cech is saving at an average rate for a keeper but chucking the ball into his own goal at a League leading rate.
I want to stress that this is a team problem. While Cech is making errors, the whole team is still not up to snuff. “10th place” is not where Arsenal’s defense should be if they want to challenge for the League. And defense isn’t just the job of the defenders. Defense starts from the front of the team and goes back. Not the other way around.
It is simplistic to put all the blame on Cech or the back four (five). The problem at Arsenal is that the entire team isn’t playing good team defense. And the result of that problem is Arsene Wenger’s worst ever Arsenal defense.
Sources: Opta, my database