Have you not seen the fight? If you watched the fight on tv like I did then it was quite clear that Tyson winced as he turned on his leg/knee........now someone like Tyson doesn't wince unless it hurts! It is quite clear that his leg/knee was on his mind from then onwards, and Williams capitalised on this, and Tyson gave up. I suggest you try to watch the full fight of TV quality broadcast on tv if you can (Not low quality Youtube video) then your question will be clearly answered.
It was the knee and knee only. U can tell where he rips it up (he asks ref for help and then later can't torque his punches and eventually can't even move out of fat slob williams lumberjack sloppy ass hayemakers)
Tyson was just well past it by then. He only had a few rounds in him at best. His knee injury would have made it hard to plant his feet and throw, much the same as for anyone with a knee, foot, ankle injury etc. In fact, probably not a great deal different from when Haye broke his toe but don't expect any unbiased opinion on that since one is a knee injury from an ATG, the other is a foot injury from a much derided figure (and rightly so).
Tyson pulled a Don King and bet against himself to make some fast cash. I don't blame him one bit either. He did the same against Brian Nelson and made even more money. With him owing the IRS and every other slimy lawyer, he doesn't give a fuk, and neither would I.
I believe the knee injury was legit, you could clearly see Tyson buckle in the 2nd round. Though it's also fair to say that Tyson had no stamina at that stage in his career.