The bout will take place at Selland Arena in Fresno, Calif.
Mayweather (29-0, 20 KOs), who held the WBC super featherweight title
for four years before moving up to lightweight in 2002, needs an impressive performance. Many thought he would continue his dominance in a heavier weight class, but it has not happened.
He did not land many punches and recorded a controversial unanimous decision over Jose Luis Castillo of Mexico to capture the 135-pound belt last April 20.
In the rematch with Castillo on Dec. 7, Mayweather used his superior foot speed to record a tight unanimous decision. While many felt Mayweather did win the rematch, he did so without displaying the skills that made him a marquee attraction as a super featherweight.
Despite questions about his power at a higher weight, the 5-8 Mayweather already is set on moving up to 154 pounds to challenge WBC and WBA super welterweight champion Oscar de la Hoya.
"I'm looking forward to fighting Juan Lazcano most likely after this fight, then fighting a tuneup at 154 and then Oscar," Mayweather said in a conference call. "I'm a throwback fighter, back in the days fighters did it all the time, move from lightweight to welterweight, from welterweight to middleweight."
For Mayweather to get anyone to believe he can fight de la Hoya, who is expected to defend his belts against Shane Mosley in September, he will need an impressive performance against Sosa.
Sosa (35-2-2, 26 KOs), the cousin of Chicago Cubs superstar Sammy Sosa, is 11
"I feel he's in the top 10, he's a solid opponent," Mayweather said of Sosa. "I feel that he beat Paul Spadafora. I feel the fight should've been stopped. It was a long count."
Sosa turned 29 on Thursday, He had a sixth-round TKO in his last fight on Feb. 21, and is fighting in the United States for just the third time.
On the undercard, Miguel Cotto (14-0, 11 KOs) of Puerto Rico, a top junior welterweight prospect, faces Texas native Joel Perez (34-5-2, 20 KOs) in a 12-round bout.