Arsenal defender Per Mertesacker has been ruled out for the 'long term' having undergone an ankle operation.

Koscielny to miss Sunderland trip, but could be back for derby day

Crisis

Jenkinson return

Arsenal's week has gone from bad to worse after defender Per Mertesacker was ruled out for the 'long term' having undergone an ankle operation to reconstruct ligaments. The German centre-back sustained the injury in last weekend's 2-1 victory at Sunderland in the build up to the hosts' opening goal. Mertesacker has yet to truly convince since moving to Arsenal last summer in an £8million deal from Werder Bremen, but his serious injury is a new setback for Arsene Wenger. The Gunners boss has seen his defence ravaged by injury problems in their battle to finish in the top four of the Premier League and has had to experiment with a range of players and line-ups. Mertesacker missed Wednesday's UEFA Champions League humiliation against AC Milan and, in his absence, Laurent Koscielny also limped out of the horror show at the San Siro.The injuries are the latest crisis for Wenger to deal with ahead of a return to Sunderland in the FA Cup this weekend and on Friday he confirmed Mertesacker has gone under the knife. "Unfortunately he has had surgery and we have lost him for a while," the Gunners boss told his club's. "How long I don't know. He had reconstruction of his ligaments, so it will be long term." Koscielny had to be substituted before half-time against Milan having suffered a knee problem which means he will not make the trip to Sunderland. But an assessment on Thursday revealed no serious damage and Wenger hopes the centre-back will be fit for next Sunday's North London derby against Tottenham."He had a scan yesterday and the news is quite good," said Wenger. "He will be out for hopefully around a week. "He will miss the Sunderland game. But he should be back for next week." Kieran Gibbs played 66 minutes against Milan and Wenger believes the full-back suffered no adverse reaction having been out since October with a groin problem. Arsenal will also be boosted on Monday by the return to training of 20-year-old defender Carl Jenkinson following his recovery from a back problem.