According to ESPN, Tiger suffered a stress fracture in his already surgically repaired left knee about a week or two before the US Open. This would explain the large amount of pain he was experiencing during the tournament.
The Golf Channel is reporting that he will also have minor surgery on his ACL. He will still fulfill his obligation to sponsors at the Buick Invitational next week as well as at the British Open.
How good is Tiger? He STILL won the US Open while walking around and putting massive amounts of torque on a stress fracture and a bum ACL. He's the f*ckin man.