First of all I am as big of a Pacquiao supporter as anyone on here, people said I was a nuthugger, but now that Pacquiao has emphatically proven himself an ATG, at least top 15 ever. After watching both Mayweather-marquez and Pacquiao-Cotto live, I know that Manny won't beat Floyd.
1st. Manny showed something I didn't think he had, the ability to absorbs Cotto's power and stand there and trade, but that doesn't help him whatsoever against Mayweather. Mayweather doesn't need to hurt you to dominate you.
2nd: Mayweather comes forward against southpaws, and Manny needs space to get off his punches, but also he isn't nearly as effective going backwards. Mayweather will keep Manny on his heels and constantly land that right cross which Manny is wide open for all night long.
3rd: Speed. Now they are both the same speed, but Manny relies on being faster way more than Mayweather, thats why the faster the opponent's hands and/or feet are the harder of a time Manny has with them.
4th: Holes. I think Cotto did a good job early of showing that if you are brave enough Manny can be counter effectively because he leaves himself so wide open after he throws punches, I think Marquez demonstrated this far more accutely in their two fights. I think Judah gave Cotto less problems than Manny because Cotto can be hit, but you need to throw punches to hit somebody, and Judah doesn't throw enough punches. I think Judah in terms of landing solidly would give Mayweather more problems than Pacquiao because against Mayweather its about landing that tight left cross as a counter puncher that does the trick, and Manny doesn't have that punch. Manny whole style and stance doesn't allow for him to catch somebody coming in with the left cross effectively. He either flurries back or counters with the right hook which Floyd would easily dodge.
Conclusion: Mayweather will carefully back Pacquiao up, take a couple of rounds to get used to his speed and what he does(just like against Judah), then Floyd will pick Manny's porous defense apart. If guys like Marquez and Cotto can left cleanly on Manny then Mayweather will do it much easier than either of them, and nobody lands punches in bunches consistently against Floyd who also has a great chin if some punches get through.