I woulda called him the man had he beaten Cotto @ 147 simply because, yes Cotto was brutally beaten up by Margo (bt his victories, like it or not is alil tainted thanx to the handwrap debacle). So to me The Man would be determained by the better recent opposition @ the weight ie. 147 pounds....and who has the best resume @ the weight in recent years? Imho that belongs/belonged to Cotto...
If he definitely isn't on PED's and beat Cotto at 147 just like he did 145 then definitely he would be the man at WW. I think Cotto actually only won 1 round, to me, that shows the dominace of Pacquiao. Pacquiao always seems underrated, then wins his fight with complete ease. He is the man to beat, Floyd has a lot more to lose though!
This is actually amusing thread because you have a case of a dog biting his own tail. We got Cotto who beat Mosley, who beat Margarito, who beat Cotto. It's like A beats B, B beats C, C beats A. So the argument that Mosley is the Man at welterweight leaves a lot of debating to resolve. For Cotto, he could have lost to Margarito who might be using plaster, or he could have lost legitimately. For Mosley, it might be due to a different styles that he lost as well as there was alleged marital problems, or whatever problem he was facing at that time. Margarito will always say he beat the fighter who beat the fighter who beat me.
I don't see how anyone could suggest that there is a genuine "man" at 147 right now. Its up in the air, sure Mosley beat Margarito, but he has fought twice there in the last few years, one of which he lost (to Cotto who was destroyed by one of the other major claimants in the division). Not to mention the two other major players are likely better in a head to head sense.
I still think there isn't a legitimate Lineal WW Champ at the moment. The lineage could be established if the winners of the Floyd/Shane and Pac/Clottey matches fought for a unification.