The bout between Mosley and Mayweather will be an acid test for both boxers. Anyone who comes out winner in this fight could just be the next challenger to Manny Pacquiao's throne. If Mayweather comes out as a clear winner and 'unscathed', then he will have the nerve to challenge Pacquiao, again. On the other hand, even if he will be the winner but badly hurt from the fight, he will have to think twice before challenging Pacquiao. He chose to take on Mosley first to test himself if he can sustain power punches. The difference is Mosley is not as quick and foxy as the Pacman in the boxing ring.
For Mosley, whether he wins or not, he will still be very much considered for the next and probably the last fight of Manny Pacquiao. The money to be gained will be really big if he fights the Filipino, win or lose. Many suspect that Clottey fought Pacquiao for money because of his dismal performance. He was just a punching bag for the Pacman.
In this fight, Floyd comes out of retirement and unbeaten. Oscar dela Hoya came out of retirement to fight and trained hard for it, but he quit and was badly beaten. Shane is an active fighter and acknowledged to be a heavy puncher. Condition-wise I would be for Shane Mosley. Floyd Mayweather may either over-train or under-train that will make his condition questionable. His stamina might be back or even better but that could easily go down with powerful punches. Remember that training does not go as far as hurting or injuring the body. It is just for conditioning. In real boxing he would be hurt and could even be injured because he had not fought for quite some time.