After Mannys last few fights I don't think he has a chance. Mayweather held out long enough to take on a declining fighter. If this fight happened 4 years ago I believe Manny would of won.
I sort of agree with Mud, though I still think Manny has a chance. As much as I dont like Mayweather and I do think he's been ducking Manny, he has excellent defense and is pretty accurate.
So i think unless Manny catches and hurts him early, Mayweather will win a 12 round decision based on defense and running from Manny. If Manny can cut off the ring though he might have a chance.
TL;DR - Want Pacman to win, but I think Mayweather wins a decision.