Andy Otten will spend six weeks on the sidelines after having surgery to repair torn medial cartilage in his knee.

Otten, 23, injured his left knee in a marking contest in the first quarter against West Coast at AAMI Stadium on Saturday.

The defender was able to play out the game, but athroscopic surgery on Wednesday revealed damage to his medial cartilage. It's the same knee he had reconstructed in late 2009, but the ACL graft remains intact.

Runner-up to Daniel Rich in the 2009 NAB Rising Star Award, Otten has played 46 games since being drafted by Adelaide with pick No.27 in the 2007 AFL Draft.

He was playing only his fifth game of the season against the Eagles, and his first since round five. Otten tweeted from the recovery ward after successful surgery:

"Op went well. Surgeon very happy and hopefully back soon for a crack at the finals!"

Coach Brenton Sanderson said the extent of Otten's injury came as a surprise.

“It came out of left field for us a bit I guess. We weren’t prepared for that," Sanderson said.

“Andy went back with the flight of the ball in a marking contest, got crunched and his knee worried him a little bit for the rest of the game.

"He played the game out with it, but the surgeon found something in the scan and it’s better to have it operated on now. He'll be fine and hopefully back ready for September."