The group comprises more than 100 ringside doctors with decades of experience in handling major fights.
"We were very surprised this bout was even sanctioned and was going to be permitted to carry on," said Larry Lovelace, a doctor and the president of the organization, which is focused on preserving fighter safety. "The thing I really fear, truly fear, is that somebody's going to get really hurt in this upcoming fight."
In June, Tim Hague, 34, a mixed martial artist turned boxer, sustained fatal injuries against Adam Braidwood in Edmonton, Alberta. It was Hague's fourth professional boxing match after competing in 34 MMA fights.
While McGregor, 29, compiled a 21-3 record in mixed martial arts, in which the rules allow him to use his feet and wrestle opponents to the ground, the Nevada State Athletic Commission decided that he was simply a premier athlete who belonged in the ring with Mayweather, who is 49-0.
"If you're going to take the position that Conor has never had an amateur or professional fight, then I'm not going to change your mind," Bob Bennett, the executive director of the commission, told the Times. "If you look at him today versus Floyd Mayweather, Conor is the taller, longer, stronger, more powerful opponent. He's also a southpaw, which makes it a little more difficult for a conventional fighter. He's 12 years younger than Floyd."
Hall of Fame referee Richard Steele said he was not sure he would have sanctioned the bout.
"Here's a guy from one sport, challenging the world's best in his own sport — I really don't know how it's going to work," Steele told the Times. "McGregor can't kick. He can't elbow. He can't do nothing. Nothing that he's used to doing that makes him a great MMA fighter."
The Nevada boxing commission has a particularly large financial stake in the Mayweather-McGregor bout, the Times reported. The state receives 8 percent of the gross revenue from every ticket sold at a boxing event in Nevada, and the commission gets 25 percent of that amount.