I just rewatched the fight a couple days ago

All we hear is "If Mosley was in his prime, he would have finished Floyd".

How are people so certain of this? Mosley was never a great finisher, even in his prime. He didn't exactly gas out until a couple rounds later. He was very fresh and spry in the early rounds. Add to the fact that Floyd had a solid chin, strong lower body, and balance unmatched by any other active fighter aside from Hopkins and Ward. Most other fighters would have gone down from that shot.

I think it was nothing more than wishful, "what-if" banter from Mayweather antagonists.