If Pacman were to spar with Rigondeaux it will shatter his confidence because Rigondeaux will be outclassing him in sparring sessions. You already saw what he did to Nonito Donaire!
Contrary to popular belief Pac doesn't bring any extraordinary speed to the table. He's fast, but Mayweather is just as fast and has incredible reflexes. The speed to bother Mayweather is the speed of Khan, not Pac. The only problem for Mayweather is underestimating the length of Pac's straight left. It's longer than one would expect. That's what knocked JMM down three times, the same long left. I think that Floyd will figure that out in the first round.
Floyd can easily use feints in order to provoke Pacquiao for straight left hands. As a result, Pacquiao will overextend himself, completely unbalanced on his feet, and Floyd will land an effective counter-punch. Later in the fight, Pacquiao will be visibly less active because he'll be afraid to be countered. However, that's not how I predict a fight. If Freddy Roach is preparing his man for the fight of the century, he must work out different approaches to break down Floyd's defence. If he only stresses on Manny's left hands, that's wouldn't work.