Despite Arsenal's come from behind victory today against Sunderland, there was a negative, with Per Mertesacker being taken off by a stretcher off after apparently hurting himself on Sunderland's tractor field pitch. Initial reports from Arsenal's Twitter feed mentioned in passing a calf injury, but according to Iain Macintosh, who was at the Stadium of Light and at the post match press conference, Mertesacker has suffered an injury to his ankle ligaments.

UPDATE: Arsène Wenger has confirmed the injury in his press conference:

It was unfortunate. Mertesacker told me that he got a shot in his ankle and couldn't move at all anymore. We are a bit worried about his ankle I must say because he goes home on crutches. He has done some ligament damage, but how big that is we will have to check tomorrow. For sure he is out for Wednesday.

No timeframe has been given for Mertesacker's return, but he obviously won't be able to play against AC Milan. For the mean time, either Francis Coquelin or Kieran Gibbs will play left back and Thomas Vermaelen can play in the middle, and Johan Djourou is back from injury. Another option could be to recall Kyle Bartley, but hopefully Mertesacker just stretched his ligaments.