Mertesacker being attended to by the medical team at Sunderland.
"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."
Mertesacker collapsed at Sunderland after attempting to shield the ball from the advancing James McClean last Saturday at the Stadium of Light. It was a routine backpass from Bacary Sagna, and an equally routine maneuver from Mertesacker.
The defender had to be stretchered off the field.
Mertesacker is likely to miss Euro 2012 as well.
With both the German international and Laurent Koscielny out, the latter from a knee injury sustained at the defeat from Italian giants AC Milan, it does not bode well for Arsenal, as they return to the Stadium of Light this Saturday to decide who advances in the FA Cup competition.
Sunderland already hold the advantage as they've had a full week of rest, while Arsenal played in midweek, allowing them only two days of rest.
The Gunners, though, have to dig deep to win at Sunderland. Champions overcome difficulty and rise to challenging situations.
Meanwhile, the other injury news is that Carl Jenkinson will be back in full training on Monday. Koscielny will be available for the Tottenham Hotspurs match at the Emirates on February 26.