I've got to disagree hard with this. If he lost the fight, it might damage him only slightly, because inevitably there'd be a rematch.
Keep in mind that the people that say Floyd didn't fight anybody DON'T KNOW SHIT ABOUT BOXING.
This dude fought real fighter when he was at his true weight- people forget that lightweight was by far his best weight - and he fought Emanuel Augustus, a prime Jose Luis Castillo twice, he made Diego Corrales, who was the undisputed champ, look like a fool, he destroyed Zab Judah when he was the welterweight champ, he beat down Chop Chop Corley, he made Gatti look like a chump, and no matter what fagass De La Hoya says, he beat him soundly while he was in his prime, AND at 154 lbs.
That's beside knocking out Ricky Hatton, who was undefeated and by far the best challenger at the time, absolutely dominating JMM in a way that no one ever has (although he was way out of his weight), and absolutely whipping Shane Mosley.
C'mon. We're talking about Floyd fucking Mayweather. Win or lose to Pac, the dude is no chump, and when he was in his prime - he's definitely NOT anymore - he was more or less unbeatable.
If he loses to Manny Pacquaio, you can bet a rematch will be signed before the twelfth round's bell rings.
One thing about Floyd that nobody like to admit, but that Brother Nazim nailed, is that if you step on that dragon's tail, he starts spittin' fire. You hit him hard, he's going to come back and humiliate you.
You beat him in a fight, the first fight he's ever lost... let me tell you something, I can't imagine the fury that he would muster in training and in fighting to beat your ass because of it.
I've watched Floyd a long time, and he gets up for every fight, even the ones that it would be easy to get trapped by. Him coming back from a loss is actually EXACTLY the Floyd Mayweather I want to see - I don't know if any fighter could match the intensity he would bring to bear as a result of losing that 0.