His chin at this stage is very questionable, that much is certain. It was a Roy Jones-esque KO, from which fighters seldom return with anything better than pitiful punch resistance. The sort of KO that makes the climax of both Lewis - Rahman bouts look like flash knockdowns. No knock on a great fighter, but it is perfectly reasonable to suspect that he doesn't take the best shot anymore. Time will tell.
Pac wins that contest by far. Knocked out ****in cold, stiff as a board. Doesnt get any worse than that. Marquez isnt exactly a big puncher. Wouldnt do it to Khan.