Just because his knee had an operation doesn't mean if one movement caused pain then every move made with the joint had to be painful. I never saw him "fall to his knees" as some have said, only a sort of "limbo" stance where his knees bent and he leaned back as he shook his fist and hollered.
Here's his reaction after making the putt on the 72nd hole:
Tiger's Put On 72nd hole, 2008 US Open
I saw him grimace after good shots as well as bad, with no set pattern except it always appeared to be after he made contact and was completing his swing, so that it seemed to be that final torque on the leg, which we now know was probably the stress fractures that were hurting, not the knee. He was able to crouch behind putts, always with left knee up, get in and out of bunkers, sometimes needing a hand from Stevie. He said he would be fine, was ready to play, and judging by the outcome I believe those were honest and sincere statements.