Watch the round again. Floyd was stunned mentally more than physically. He was stunned thus because of his inability to detect and evade that punch. If you look at his face throughout, you do not see any signs of loss of awareness and position as a result of a hard punch. Yes he was hurt, but 90% of that hurt was surprise and 10% was physical.

Look at Floyd's adjustment from the 3rd round. He began giving Mosely the shoulder. That shoulder protects him from that half over hand right cross that follows a jab. That was Mosely's preferred stock in his tangle with Margarito. It became a white elephant after Floyd measured its range, and made allowance for it.