Arsene Wenger has confirmed that ankle surgery for Per Mertesacker will keep him out for the rest of the season. This news was expected after the manager had said earlier in the week that he'd be out "long-term", so now we must do with Johan Djourou, Thomas Vermaelen and Laurent Koscielny.
The loss will be a disappointment, though not devastating. While the German had excellent positional sense and a good relationship with Koscielny, Vermaelen and Koscielny, until last week against Milan, had usually been excellent together. There is a concern that both may be too similar, but over the past year, Koscielny has become superb at defending deeper, especially as Mertesacker's pace made him a little more conservative in how he went after attacking players.
The concern, of course, with Kieran Gibbs an injury prone player and Andre Santos yet to return, is that Vermaelen may be required to play left back, making Johan Djourou or even Sebastien Squillaci partner Koscielny in defence. Hopefully, Gibbs remains fit for the rest of the season, and Santos comes back, but if that isn't the case, perhaps it makes sense for Wenger to recall Kyle Bartley on loan--Bartley, being under 21, will not need to be registered for the Premier League.