Guest Post: Wenger is right, Arsenal’s defense is below average

Guest post by Scott Willis

Yesterday, I had a series of tweets on Arsenal’s defense and Petr Cech that caused a bit of a backlash against me and had people questioning my sanity. I wanted to take some time and move from twitter to a medium that actually allowed for a bit longer and a more nuanced explanation of how I arrived at these conclusions.

My main point is that Arsenal’s defense is mediocre this season and that they are more closely resembling a team fighting for a Europa League spot instead of one that is fighting for a Champions League place.

I also made the point that I thought that Petr Cech was a big reason that the goals allowed stats suggested that Arsenal are still in the conversation for top 4 and that had people questioning my eye sight and if watched Arsenal play at all.

I will try to dig through these with a little more depth and longer explanations.

First is the argument that Arsenal’s defense Is mediocre this season.

I have created a database of fairly detailed shot stats this year and one of the things that I was playing around with this morning was creating a ratio that compared different stats as a percentage of league average. Using this method is popular in baseball as a quick way of seeing how far above or below someone is above average and I thought it would be interesting to see for the Premier League.

As I did this it became very obvious that Arsenal are very unlike the other five teams above them in the table defensively.

First is the percentage of shots allowed per game compared to league average for each team.

TEAM Percent of league average
Chelsea 61%
Tottenham Hotspur 72%
Manchester City 62%
Liverpool 66%
Manchester United 68%
Arsenal 80%


Chelsea are really good at not allowing teams to get any kind of shot off against them, the other top 4 contenders are not too far off but bringing up the rear is Arsenal. Arsenal are better than average but they are not nearly on the same level as the other teams they are chasing.

Next let’s look at shots in the danger zone. The danger zone is classified as the six yard box plus the center of the 18 yard box. This is where the vast majority if goals are scored and where you really want to limit shots taken from.

TEAM Percent of league average
Chelsea 57%
Tottenham Hotspur 78%
Manchester City 60%
Liverpool 70%
Manchester United 63%
Arsenal 86%


The story is the same for this measure, Chelsea are really good at limiting shots from the danger zone. The other top teams are fairly close (except Tottenham have dropped off a bit) and allow well below the average team leaving Arsenal better than average but not in the same league as the others.

Next let’s take a look at the shots allowed on target.

TEAM Percent of league average
Chelsea 59%
Tottenham Hotspur 71%
Manchester City 64%
Liverpool 76%
Manchester United 64%
Arsenal 93%

Perhaps I should just copy and paste what I wrote the last two times down here. There is Chelsea leading the way, there is a chasing pack and then a big separation and there is Arsenal.

Next let’s move to something that gives more context to the quality of shots allowed besides the location alone and look at big chances. Big chances are an Opta stat that says that the chance was one that you would be expected to score regularly (in my data set they are scored 47% of the time)

TEAM Percent of league average
Chelsea 60%
Tottenham Hotspur 84%
Manchester City 60%
Liverpool 95%
Manchester United 54%
Arsenal 95%


This time things are different but still look bad for Arsenal. This time it is Manchester United who allow the least big chances compared to league average, followed by City and Chelsea. Tottenham is above average but not great and Arsenal and Liverpool are just about at the league average mark. This last bit could go a ways in explaining some of Liverpool’s issues; they don’t give up many shots but when they do they are high quality chances.

Lastly let’s take a look at overall chance quality allowed (what I like to call my version of xG). Chance quality takes many things into consideration, where the shot was taken, was it with feet or head, how was it assisted, was it considered a big chance and puts it all together based on the historical weights each of these have on scoring chances.

TEAM Percent of league average
Chelsea 55%
Tottenham Hotspur 78%
Manchester City 61%
Liverpool 79%
Manchester United 62%
Arsenal 85%


Unsurprisingly after looking at the previous numbers Chelsea come out the leaders followed by the teams from Manchester and then Tottenham and Liverpool with Arsenal bringing up the rear with a number that suggests they have the 7th best defense in the league the same distance to Everton in 8th as Liverpool in 6th.

After looking at these numbers I think that I rightly concluded that Arsenal do not have a defense that normal title contenders have, let alone a defense that you would expect to be able to challenge for a top four spot with six other strong teams. That conclusion is further buttressed by watching the team on a regular basis where it has become apparent that there are some major structural issues with the way that Arsenal defend over the last couple of seasons.

That brings me to the second part of the series of tweets that seemed to have people questioning my sanity; Petr Cech is a large reason why the top line goals allowed numbers don’t look too bad. Arsenal have allowed 27 non-penalty goals this season which is 4th best. This is 8 goals behind Chelsea, Tottenham and Manchester United who are all at 19 but ahead of both Liverpool and Manchester City who have much better shot based stats than Arsenal.

Doing the same kind of analysis as before but using simply the non-penalty goals allowed as a percentage of league average you get the following table.

TEAM Percent of league average
Chelsea 51%
Tottenham Hotspur 51%
Manchester City 77%
Liverpool 94%
Manchester United 51%
Arsenal 74%


Arsenal are right in the thick of things now with the 4th best mark. Chelsea still lead the way tied with Tottenham and Manchester United. Manchester City is in 5th well behind what the other stats suggest and bringing up the rear in Liverpool. For all of these teams you can construct a solid narrative about their position on the table compared to their stats based on their goal keeper.

Chelsea have a solid defense and the numbers are improved by having Thibaut Courtois in goal. Tottenham have good stats but that is elevated into an elite defense at prevent goals by Hugo Lloris. Manchester United have adapted to Jose Mourinho’s defensive minded ways and have a good defensive unit that is again elevated David de Gea in goal. Manchester City have good defensive numbers but these have been undermined by the goalkeeping errors of Claudio Bravo and the same can be written of Liverpool and their rotating duo of Loris Karius and Simon Mignolet.

I essentially did the same thing with Arsenal, I saw that they had pretty mediocre stats yet the overall numbers looked fairly decent and came to the conclusion that Petr Cech is actually a positive influence on Arsenal’s overall defensive numbers.

I do want to defend that a bit more but first I want to illustrate some limitations in the data I have above. That does not include penalties, normally I don’t include penalties in when I am looking at offensive stats because that is a different skill than when you are looking at scoring but that is a major thing that you want a goal keeper to be good at stopping. Cech has faced 7 penalties this season and stopped none of them while looking like he had no chance on almost all of them and that should be held against him. The second part is that goalkeeping stats are very crude, it is hard to assign credit/blame to a goalkeeper compared to his defense. It is also hard to examine things without knowing where a shot was placed and knowing things like his positioning.

All of those caveats out the way the numbers do suggest that Cech that while he is no longer among the elite he is still a good shot stopper as long as the shot isn’t a penalty. Arsenal (and with Cech playing the vast majority of the minutes) have the 3rd best save percentage behind Tottenham and Burnley. Arsenal also have the 5th best expected goal efficiency which could hint at a good goal keeper helping to keep more goals out than expected.

All of that said, I too can see that Cech is on the downside of his career and that it looks like he has taken a step back this season. I would be happy to see a new keeper come into Arsenal next season to take his place but if that does happen I would warn against putting too high of expectations that it will fix things without a change to the defense in front of the goalkeeper.

Scott Willis tweets at Oh That Crab


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