Illinois (3-4) announced Monday that the fifth-year center was injured in Saturday's loss to then-No. 4 Iowa State. Thorne had surgery Monday.
Thorne was averaging 13.4 points and 8.4 rebounds a game. He is the team's leading rebounder, playing 22.1 minutes a game.
Thorne has started all seven games for Illinois, which played the Cyclones evenly before he left and eventually lost 84-73.
The Illini host Notre Dame on Wednesday.