The Westgate SuperBook installed McGregor as -1,000 favorite Monday over Mayweather, who opened as a +650 underdog. The sportsbook mandates that the fight must be MMA only and take place before Sept. 1, 2019.

Mayweather defeated McGregor in August in a boxing match that generated massive betting interest. Nevada officials estimated $65 million was bet on Mayweather-McGregor, the most ever for a boxing match, and sportsbooks in the United Kingdom also reported record betting action on the first clash between the two sports' brightest stars.

Mayweather (50-0, 27 KOs) was a big favorite over McGregor in the boxing ring, closing at around -500 odds at most sportsbooks. That bout had also opened at Westgate as a hypothetical, in February 2017, with Mayweather as the -2500 favorite and McGregor the +1100.

Mayweather, 41, would be expected to be a big underdog against any MMA fighter, let alone against McGregor, a two-time UFC champion.

Even so, Mayweather has maintained that he is taking a future in MMA seriously and recently told TMZ Sports that he plans to apply for a license.

"Everything takes time," Mayweather told TMZ Sports. "Eventually, we're gonna apply for the license, and hopefully we can fight."

Betting limits on the hypothetical match are $2,000 at the Westgate.