It will come as a blow to Gunners boss Arsene Wenger in a week that saw them go down to their worst defeat in Europe.
“Unfortunately he has had surgery and we have lost him for a while. How long I don’t know. He had reconstruction of his ligaments. So it will be long term,” Wenger told the Arsenal official website.
It means that Wenger will be forced to pursue with a central defensive partnership of Johan Djourou and Thomas Vermaelen for tomorrow’s FA Cup fifth round clash with Sunderland, as Laurent Koscielny is also out.
The French centre-half was removed before half time with knee trouble in the 4-0 defeat at Milan, but news on him is positive and he is expected to return for the Tottenham game next weekend.
“He had a scan yesterday and the news is quite good. He will be out for hopefully around a week. He will miss the Sunderland game. But he should be back for next week,” added the Emirates Stadium chief.
Despite many criticising Mertesacker for his lack of pace since coming to the Premier League, his reading of the game and organisational skills played a big part in Arsenal’s recovery from their disastrous early period this season.
There is also better news concerning Arsenal’s full-back situation as Kieran Gibbs did not feel any ill effects from the Milan game and Carl Jenkinson is due to start full training again on Monday.