his knee hurt but he also got punched the same way he got punched against Kongo, he recovered very quick
The knee issue came because the leg buckled in a bad angle when Cain's lights went out for half a second
the buckling of the leg, caused by the uppercut, was the cause for the knee injury. IT DIDNT HAPPEN BY FLUKE or by slip.
Cain got punched, lights out for half a second like in the Kongo fight, knee buckled in the wrong way, further injured his leg. Cain doesnt remember the punch, so he thinks its just the knee that gave out. He woke up as his leg was buckling, so he woke up to knee pain
see how fast cain recovers