Ahead of the highly anticipated superstar bout between two contact sports legends, boxing champion Floyd Mayweather labelled his opponent Conor McGregor a "dirty" fighter after seeing footage of one of his sparring sessions.

"I see a lot of rabbit punches behind the head, grappling, wrestling, illegal shots," Mayweather said, according to a report from USA Today. One of McGregor's challenges, aside from out boxing his proven opponent, is keeping his fight within the rules of boxing.

The Ultimate Fighting Championship (UFC) lightweight champion has always fought in the more diverse field of Mixed Martial Arts, which features a lot of fighting techniques aside from boxing. Although he has shown excellent stand up fighting skills during his UFC matches, sticking to boxing while reining in the rest of his combat skills would be a tough challenge for McGregor.

In line with this, the Irish fighter has hired retired Hall of Fame referee Joe Cortez to oversee his training and make sure he adapts to boxing rules. McGregor's former sparring partner Paulie Malignaggi has expressed concern over McGregor's ability to stay within the rules.

"His intensity is a good thing, I just don't know if he can go without fouling," Malignaggi said, according to BBC. "Fighting up close, I think he gets a bit confused and he ends up fouling. I think that's where he can be most intense but fighting up close is the hardest thing in boxing to learn and I don't think you can do that in a few months."

Veteran referee Robert Byrd has been chosen to oversee the fight, which will see Mayweather come out of retirement to welcome UFC sensation McGregor into the boxing world.

Despite worries about keeping the Aug. 26 bout clean, the 40-year old boxing icon is confident that referee Byrd will be able to control the bout. "The ref will be fair on both sides, I want him to be even. I want us to have a good solid fight," said Mayweather.