Per Mertesacker was sent off against Chelsea, and has not played a minute of football since. Mertesacker served his ban during Arsenal's 2-1 victory against Burnley in the FA Cup, but Arsène Wenger kept faith in the partnership of Gabriel and Laurent Koscielny for successive league games against Southampton and Bournemouth.
Clean sheets were kept in both, though that ridiculously simplifies the point, not least because Héctor Bellerín was called upon to make a last-ditch tackle after Marc Pugh capitalised on a Gabriel error, and Petr Cech made several impressive saves. Perhaps Wenger was sticking with the partnership because of the idea of the hot hand, perhaps he has been punishing Mertesacker, or perhaps he is thinking of utilising Gabriel's pace against Leicester City and Jamie Vardy on Sunday. All three are short-sighted.
While Mertesacker is partially culpable for his red card, to apportion all the blame to him is simply unfair, or the work of someone with an agenda. There are several errors in the build-up to the red card. The first error is Mathieu Flamini, who is quite simply awful, failing to close Willian down, effectively waving him as if he were marshalling an aircraft. The second mistake comes from Laurent Koscielny, who never looks to see if Mertesacker is attempting to play offside, steps up, allowing Costa to make the run. There is an argument Mertesacker should let Costa go--after all, 11 men when chasing a goal is better than 10, and he would have to beat Petr Cech--but that is a hard decision to make while in the heat of the moment.
The idea of Wenger sticking with Gabriel and Koscielny because of the hot-hand also seems bizarre. Gabriel is a promising defender, but at the moment, his strengths consist of being strong in the tackle and quick. He is weak aerially, being at fault for Burnley's equaliser as he was easily beaten by Sam Vokes, and his passing from the back is atrocious. This is perhaps no better emphasized than a passage of play last Sunday, when Aaron Ramsey and Héctor Bellerín worked the ball out of pressure, but only had Gabriel and Flamini open. The two passed between themselves like they were dealing with a hot potato before Gabriel, under minimal pressure, hit an aimless long ball. This would be less of an issue if Arsenal had a controlling midfielder in the side, but unless Mohamed Elneny is trusted to be that player, or Mikel Arteta is fit enough to start, Arsenal simply do not have such a player. This affects the build-up play; Arsenal are unable to move the ball quickly back to front, and instead hit the ball longer.
Mertesacker, of course, is good at building play, and his partnership with Koscielny excels in that area simply because he plays shorter, tempo setting passes, while Koscielny plays between the lines. Both are needed; without one, Arsenal are too direct, or unable to move the ball into threatening areas.
Finally, there is the question of pace. With attacks on Mertesacker's pace, or lack there of, one would think that Mertesacker's pace cost Arsenal goals every week. That, of course, is not true, and there is a litany of examples where Mertesacker has not been caught out by pace, cases that far exceed those where he has. There are two reasons for this: the first is organisation, and the second is positioning. Knowledge of positioning, of where to stand, far exceeds pace in importance, as does communication. This is an area where Mertesacker and Koscielny, as a partnership, are usually quite strong. Mertesacker's positioning and anticipation are far ahead of that of Gabriel, who despite his pace can still be caught out--as he was last weekend.
Ultimately, the best centre back partnerships are those where the attributes of both are complementary. The partnership of Gabriel and Koscielny is not that; Gabriel, like Koscielny, is impetuous and occasionally rash, preferring to step up and nick the ball. Arsenal had a similar situation 3 seasons ago, when Laurent Koscielny frequently partnered Thomas Vermaelen, until the mistakes of the partnership caused the manager to revert to Mertesacker, despite the abundance of pace in the Koscielny/Vermaelen partnership. This is because, ultimately, intelligence and positioning trumps raw athletic attributes. Wenger would do well to remember that this weekend while facing a team that prides itself on athleticism.