Because his knee was injured. Not feeling any pain or not feeling as much as you should doesn't mean that you don't feel that something is wrong.
my sister was skating in france, dropped badly cut up and everything on her knee. after 3 days she gets on the plane home sits down and feels her other leg felt weird so went to the doctor.....cruciate ligament snapped, 6 surgery's later she cant walk longer than 7 mins. she was a silver medalist in kickboxing style ITF taekwon-do. now cant do nish. so yeah. just because it isn't a bloody, obvious injury doesn't mean it was debilitating. he probably didn't know what was going on until he stood up right and the ligament wasn't there to support his weight.
I think what happened was he lost his motor skills from the punch. His Glass Chin is so delicate that one little peck from the Iron fist of Vitali made it crack. Solis' brain was jarred , therefore losing all balance and motor skills - which leads to the Glass Knee. Wow, what dedicated work to study this official Glass Dome of Solis. That KO was a ChinChecker Checkmate if I ever saw one!
Hmm....:huh I think he just was in a REM sleep by the time he reached for his knee actually. It was like, Punch...delayed response...falls over, grabs knee while falling.
And some "fans" still complain that Vitali quit against Byrd because he had a torn rotator cuff, but it's OK is Solis quits with no pain :huh