If there is one criticism that remains consistent throughout Arsène Wenger's time at Arsenal, it is that he is remarkably conservative, and defers to seniority over young talent. It seems contradictory with the ideals of Project Youth and Wenger's reputation, but Project Youth was a financially enforced choice, and young players during Wenger's reign--such as Nicholas Anelka or Cesc Fabregas--only played through other's absences, and proved to be so talented that the senior player was eventually discarded (Ian Wright in Anelka's case, Patrick Vieira in Fabregas').
Calum Chambers is not, at present, at the level of Nicholas Anelka or Cesc Fabregas. Yet, the player he covered for against Bournemouth, Mathieu Flamini, is not at the level of Ian Wright or Patrick Vieira. Rather than being a superstar, Flamini is a 31-year old third choice defensive midfielder, who Arsenal were prepared to cast out last summer. Continual injury to Mikel Arteta and injury to Francis Coquelin has given Flamini a place in the first team again, not out of merit, but out of purely being around and having two functioning legs. The performances, while not disastrous, have also not been satisfactory, and the limitations are being exposed, even by sides as fundamentally bad as Newcastle. Flamini may be a green energy tycoon, but he is still as problematic as he was last season, and is not getting any better.
The issue is not just defensively. When told what to do, Flamini tends to do it fairly well, if somewhat haphazardly, and he did a fine job of keeping David Silva quiet when Manchester City came to the Emirates. The bigger problem is when Arsenal are on the ball: Flamini is limited in build-up play, often looking to make a decoy run. But Aaron Ramsey does less building in deeper areas, and thus, Arsenal's supply to Mesut Özil can be cut off, as it was against Southampton and Newcastle. Ramsey could, of course, take more responsibility in building play, and there was a sense of him doing that in the second half against Newcastle. But it is not his natural game to play as deep as Flamini. The other solution is a holding midfielder who can pass the ball well, and there is hope that Mohamed Elneny, once he joins and gets acclimated to Arsenal and the Premier League, can take that role. But Arsenal have another solution in Calum Chambers, and after his display against Bournemouth, he was unlucky to be dropped.
The difference is quite striking, and is well portrayed by @11tegen11's passing maps:
Against Bournemouth, the presence and ability of Chambers allowed Ramsey to play higher up and combine with Özil, allowing Özil to dominate the match. Against Newcastle, Arsenal struggled in possession, and Özil was often marked out of the game. For Arsenal to be successful in coming matches against Liverpool, Stoke and Chelsea, Özil will have to be given the platform to run matches, a platform that is far better with Calum Chambers in central midfield than Mathieu Flamini.