Floyd Mayweather says footage of Conor McGregor sparring Paulie Malignaggi has shown the UFC lightweight champion to be "extremely dirty" in the ring.
Malignaggi left McGregor's camp when images of him down on the canvas were made public after the session.
"A lot of shots were illegal," said Mayweather. "Lots of grappling and illegal punching behind the head."
On Thursday, Malignaggi told BBC Sport he questioned if McGregor could "keep it clean" against Mayweather.
"His intensity is a good thing, I just don't know if he can go without fouling," Malignaggi told 5 live's boxing podcast. "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."
McGregor, 29, has employed retired Hall of Fame referee Joe Cortez in order to adapt to boxing rules for his debut in the sport against undefeated Mayweather, who will come out of retirement for the 26 August bout in Las Vegas.
Mayweather, speaking to a teleconference of journalists around the world, said: "He had Joe Cortez in his camp but I still saw him being extremely dirty. My job is not to worry about the referee but to let the referee do his job."
Robert Byrd, himself a Hall of Fame referee, has been chosen to officiate the T-Mobile Arena bout.
'My last fight and I'm the risk taker'
Mayweather, 40, will surpass Rocky Marciano's perfect record of 49 fights without defeat should he avoid loss against McGregor. The Las Vegas-based fighter retired in 2015 but is adamant this will now be his final contest, stating he has "given my word" to his family and advisor Al Haymon.
He is a heavy favourite with bookmakers to score a 50th win almost 21 years since his professional debut. On Wednesday, McGregor said he expected to win inside two rounds.
Speaking to BBC Sport during Thursday's teleconference, Mayweather added: "I truly believe I am taking a bigger risk but it's a big reward for both of us. I have the 49-0 record. When a fighter has been dominant for 20 odd years, everything is on the line.
"I just try to focus on being positive. I try not to think about taking a loss. Every day I tell myself I'm a winner. I was born to be a winner in life. Whatever I do I try to give 100% and go to the limit.
"It's all about taking a risk and I wouldn't be where I'm at. I don't mind putting the 49 and 0 on the line, I think it's worth it."
Analysis - 'Mayweather moves in the mirror?'
BBC Radio 5 live boxing analyst Steve Bunce:
It's going to be a relentless week. Too many people are treating it like a comedy carnival. On the night I think it will be a lot more serious than people think. I think the laughter will stop in Las Vegas, probably around Wednesday.
McGregor is a guy who was nowhere four years ago. Now he can earn hundreds of millions of dollars. He got there because he is absolutely driven. I've seen his fights, I think he likes hurting people. So there will be dark undertones next week. Anyone who thinks Mayweather doesn't have a nasty, dark side has been living in a cave for 30 years.
I believe McGregor has picked up a lot from Mayweather. If you are skint like he was five or six years ago and you are seeing a guy like Mayweather who has been making it rain throwing dollar bills in the air, of course he is copying that. He has wanted a bit of that. I can imagine McGregor before he goes to bed at night doing a few Mayweather moves in the mirror.
The Making of McGregor will be broadcast on BBC Radio 5 live on Friday, 18 August at 21:00 BST.