Mayweather told reporters on a conference call on Thursday he believed mixed martial arts star McGregor had used illegal tactics.
Mayweather told reporters on a conference call on Thursday he believed mixed martial arts star McGregor had used illegal tactics in sparring sessions.
The undefeated former welterweight king, who is coming out of retirement to fight McGregor in Las Vegas, said he had spotted rabbit punches in footage of McGregor's sparring session with Paulie Malignaggi.
McGregor, who has never fought in a professional boxing contest, appears to floor Malignaggi in the footage.
Mayweather, 40, was unimpressed.
"I had a chance to see it and my thought were, it was interesting," Mayweather said of McGregor's Malignaggi knockdown.
"A lot of rabbit punching, a lot of illegal shots, behind the head," he added.
McGregor has hired veteran referee Joe Cortez during his training camp to help advise on boxing rules and ensure he fights clean.
"He's had Joe Cortez in his training camp and I've still seen him being extremely dirty," Mayweather said.
Veteran referee Robert Byrd will be responsible for managing the fight and Mayweather is confident he will maintain control of the contest. Byrd officiated in Mayweather's 2013 win over Robert Guerrero in Las Vegas.
"I truly believe the referee is going to do a great job," Mayweather said.
"The referee's job is to keep the fight clean. He don't have anything to worry about. I'm looking forward to following the Queensberry rules of boxing," he added.
Mayweather retired from boxing in 2015 with a perfect 49-0 record to emulate former heavyweight legend Rocky Marciano. A win against McGregor would see the welterweight become the only man to ever hold a perfect 50-0 record.
Mayweather insisted however that moving clear of Marciano's historic benchmark was not on his mind.
"Even though this is my 50th fight, this is not my focus," Mayweather said. "Rocky Marciano is a legend, he did it his way. I just like to do it the Mayweather way."
Win or lose, Mayweather is adamant that nothing will be able to tempt him back into the ring following his return against McGregor. He reiterated an earlier statement that this month's fight will be his last.
"This is my last one. I gave my word to (adviser) Al Haymon, I gave my word to my children, I don't want to break that," Mayweather said.
"I'm going to stick to my word -- this is going to be my last fight."