Arsenal defender Per Mertesacker has undergone ligament reconstruction surgery on his ankle and now faces a lengthy spell on the sidelines.

Mertesacker, 27, suffered the injury in last week's 2-1 win over Sunderland, another blow to an Arsenal defence already ravaged by injuries.

Laurent Koscielny was forced to limp out of his side's 4-0 loss to AC Milan in midweek, but his injury is not expected to be as bad as Mertesacker's, who manager Arsene Wenger fears will be out for a long time.

"Unfortunately he has had surgery and we have lost him for a while," Wenger told the club's official website.

"How long I don't know. He had reconstruction of his ligaments, so it will be long term."

Arsenal currently sit fourth in the Premier League, ten points behind North London rivals Tottenham.