Simmons grabbed a rebound and appeared to tweak his knee. He left the game and hobbled toward the locker room. He left the locker late in the fourth quarter wearing street clothes.
Coach Brett Brown said after the game the team doesn't know anything on the extent of the injury.
''We don't know,'' Brown said. ''I'm sure the club will make some announcement. But as far as knowing more than that, I don't. Obviously I'm very curious.''
Simmons had eight points on 2-of-10 shooting before his departure.
Philadelphia is off Thursday before facing Orlando on Friday.