Not necessarily. Even if he fought Floyd in May and lost, there's still plenty of good money making fights for Manny, including a 4th fight with Marquez. There have been a lot of top fighters who lost big fights and it didn't end their careers, from Robinson to Ali to Leonard to Hearns (who lost both of his biggest fights) to Trinidad (same) to De La Hoya.
I disagree. Only Marquez comes out of this unscathed. Pac will lose an embarrassing fight to Mayweather. Every "fan"(casual) of the sport who jumped on the Manny bandwagon will see that he is nowhere near the level of a Floyd Mayweather. It will be obvious that Top Rank had been ducking Floyd because they knew what was going to happen, instead they gave us such classic match ups as Clottey/Pacman and Clottey/Margs. People will know what's up. Not saying Manny's career would be over, as you said, there is always Pac/JMM 4 but Pac's time as the reigning king of the sport will be DEAD. What bullshit would they have in stock to sell us then? Who's gonna pick up the Top Rank flag after that - Bradley?!?! Cotto?!?! I'm sure they'd start to focus on Donaire which would be terrible because they won't put him in with any threats just like Manny. Case in point, Donaire vs Vazquez.
