Barring severe physical advantage from mayweather on fight night a decision is in order. An interesting thing about the reach of both pacquiao and marquez. In the pac diaz fight pacquiao reach is 25" then in dlh pac its 23". Same with marquez, in some fights hes 21" then hes matching manny in others. I wouldnt be surprised if mayweather is only slightly bigger than marquez on fight night. Plus the delay time give marquez more time to slowly put lean muscle on for the fight.
Potentially, yep. Even someone who hasn't been closely monitoring the situation could see the effect of the punches on Marquez coming from normally average to light hitting Juan Diaz at 135. Anything can happen, but if Mayweather can't stop Marquez at this point in Juan Manuel's career, at this weight, he's underachieving.