By Miguel Rivera

Former four division world champion Juan Manuel Marquez is firmly against the potential cross-sports fight between Floyd Mayweather Jr. and UFC superstar Conor McGregor.

The two athletes have been baiting each other for a potential fight for the last year.  

In the last few weeks, their discussions have become more serious. Mayweather (49-0, 26 KOs) announced that he's officially unretired - but only to go forward with a potential fight against McGregor. Mayweather, a former five division world champion, retired from the sport in September 2015 after defeating Andre Berto.

McGregor has started training for the fight and posted several videos that showed him sparring and doing numerous boxing related workouts.

Marquez knows Mayweather better than most. In 2009, he was dominated over twelve rounds by the former pound-for-pound king.

Marquez, like many others, believes the fight would be a severe mismatch if McGregor got in the ring to face Mayweather in a boxing match.

"It's an insult to the sport and an insult to boxing and to the UFC . Every sport deserves respect and in this case boxing and the UFC deserve it," said Marquez to the media in Mexico City.

"You can not do a fight like that. It's like boxing a soccer player or a basketball player or vice versa, it's impossible [for McGregor to win that fight]."

Before McGregor and Mayweather can go forward with such a fight, the UFC would have to be on board. So far, the organization has expressed little to no interest in allowing their biggest star to move forward with a contest against Mayweather - at least under the rules of a boxing match.

McGregor, who obtained a boxing license in California, would also need to secure a license in Nevada, as Mayweather would never fight outside of Las Vegas.

Marquez, 43 years old, is pondering the possibility of returning to the ring this year. He's been inactive since May of 2014, when he won a twelve round unanimous decision over Mike Alvarado.