2011-2012 Premier League Arsenal Mertesacker
Image credit: dpa
Arsenal initially ruled the German out for a month from the injury he sustained during the Premier League victory at Sunderland.
Wenger said in his Friday press conference that following the reconstructive surgery it was unclear when the defender would return to action.
"Unfortunately he has had surgery and we have lost him for a while," Wenger said. "How long I don’t know. He had reconstruction of his ligaments. So it will be long term."
The injury makes him a doubt for Germany's Euro 2012 campaign. Mertesacker has 79 caps at international level at the age of 27.
Mertesacker has made 22 appearances for the Gunners since joining from Werder Bremen for an £8 million fee on deadline day of last summer's transfer window.