PITTSBURGH (AP) Pittsburgh Steelers linebacker James Harrison underwent surgery on his left knee Wednesday and his return is uncertain.

The 34-year-old Harrison has been struggling with lingering pain in the knee for months. Coach Mike Tomlin called it a ``minor'' procedure but added he's not ready to put a timetable on when Harrison will be cleared to play.

Harrison is currently on the physically unable to perform list. He sat out the preseason opener against Philadelphia last week and it was unlikely he was going to play in Sunday's game with Indianapolis even before the surgery. Tomlin said the team deemed the procedure ``necessary'' but wouldn't call it a setback.

The knee limited Harrison during Organized Team Activities. With Harrison out, second-year linebacker Chris Carter will likely start at outside linebacker against the Colts.