The real reason why Floyd has been avoiding southpaws is..

As Mosely pointed out in the pre-fight hbo legends clip. Floyd's shoulder roll is open for a left straight down the pipe. Roach insisted that Mayweather's shoulder roll will not work against southpaws like Pacquiao, and even warned doom for the flamboyant American if he utilizes the same defensive tactic come fight night. It's so overrated. It doesn't work against southpaws, if he does do it; he will have to pay for it, said Roach. I truly believe that Floyd having not to use his shoulder roll when fighting a southpaw becomes less of a complete fighter than when he is fighting an orthodox fighter. Without his shoulder roll he is not 100% Floyd Mayweather. Without Floyd's shoulder roll in play he cannot be the complete defensive genius that we are known to see. His shoulder roll is not only used for his defense, but he uses his shoulder roll to set up most of his offense. Without being able to use it it becomes a disadvantage for him knowing how much a role it plays in his game. It's one of his signature moves that he relies on and if he can't use it against the likes of an accurate southpaw that has a deadly left straight. Floyd will be limited without his shoulder roll. He can use other skills in his bag of tricks but he can't use his main weapon of choice which has been and always been the shoulder roll.