In 2012 Bob Arum — the Top Rank boss who promotes Pacquiao and was Mayweather's promoter between 1996 and 2007 — said Floyd turned down $100 million to fight Pacquiao because he hates southpaws.
Pacquiao's trainer Freddie Roach said something similar in an interview with SI's Greg Bishop. Roach explained that Mayweather's patented defense — where he guards his body with his left arm, deflects punches with his left shoulder, and blocks/counters with his right hand — won't be as dominant against Pacquiao because he's a southpaw.
it's certainly an angle to consider. here's a synopsis of his southpaw fights:
Despite his father’s claims, Mayweather can’t deny his difficulty with southpaws. His first left-handed opponent Reggie Sanders was a headache. Mayweather was cut near the eye and booed by the crowd as his punches were wide, tentative and slow. Boxing News said he was a disappointment in his second pro fight in 1996.
His second southpaw victim Bobby Giepert, a patsy, was decked twice in losing in 90 seconds in 1997.
Mayweather was cut over the left eye in a surprisingly tough fight against his third left-handed victim Jesus Chavez in 1997. Chavez kept Mayweather off-balance by sliding to his right in the early going. In the fifth, Mayweather landed a right lead capped by a left hook to end it.
Corley was Mayweather’s fourth southpaw opponent and proved tough as nails in 2004. Mayweather bled from the nose and mouth, took a lot of punishment and couldn’t put Corley away to settle for a win on points.
Fifth on the list was Mitchell who landed a solid left to knock Mayweather back on his heels in the second round in 2005. But Mitchell was way past his prime and eventually wilted to lose by stoppage in the sixth. Mayweather did some switch-hitting against Mitchell, a trick he’ll likely use against Pacquiao.
Judah was the sixth southpaw victim in 2006. In a tense moment, Judah connected with a right hook that wobbled Mayweather who put his right hand on the canvas to keep steady. Referee Richard Steele ignored it and didn’t rule a knockdown. A melee erupted in the 10th round as Money’s uncle and trainer Roger Mayweather entered the ring to confront Steele after Judah threw a low blow and landed a rabbit punch. Security staff, boxing officials and Judah’s father Yoel joined the protagonists in the ring. When order was restored, Mayweather breezed to a win on points.
Victor Ortiz was Mayweather’s seventh left-handed victim in 2011. In the fourth round, Ortiz pounded Mayweather’s head with a hard left. Money backtracked with Ortiz on the attack. Ortiz lost his composure in trying to score a knockout and in exasperation, jumped to butt Mayweather in the mouth. Ortiz acknowledged the foul and appeared to show remorse by attempting to kiss Mayweather on the cheek. Referee Joe Cortez led Ortiz away by the hand and gestured to deduct a point. As Cortez waved them back in, Ortiz was still apologetic and Mayweather took advantage to unleash a left hook. Ortiz looked at Cortez as if to solicit sympathy and on his blind side, was whacked by a right cross. The sucker punch brought the curtains down on Ortiz in the fourth.
Finally, there was Robert (The Ghost) Guerrero, Mayweather’s eighth southpaw victim. It was a lopsided victory although Guerrero never went down in their 2013 fight. Mayweather turned Guerrero into a punching bag, blasting his head and body with precision, leaving the Ghost lunging at shadows and was masterfully in command from start to finish against a poor excuse for an opponent.
Mayweather’s record against eight southpaw victims is 8-0, with 4 KOs. It’s no coincidence that less than 20 percent of his opponents were left-handers. Neither Mayweather nor his father has been known to be fond of southpaws. On May 2, Mayweather faces probably the toughest left-hander he’ll ever meet, someone who could prove his allergy to southpaws is no myth.
This is another way of saying Pacquiao will have to control the tempo however possible including this Southpaw angle to make it his fight or Money's quickness will win out.
There is no secret that Floyd have a hard time against southpaws. In fact, every orthodox fighter have a hard time fighting southpaws. But Floyd will be ready for this one. Pretty sure they studied this one carefully. That's why he decided to fight Pac now in the first place.