Dana White tells TMZ Sports ... if Floyd Mayweather is serious about fighting Khabib Nurmagomedov, it ain't gonna be a boxing match -- it's the Octagon or nothing!
Floyd has said he's 70% certain the two sides will get a deal done to box -- he expects to make a 9-FIGURE CHECK if it comes to fruition.
But, when we saw Dana at Mr. Chow in Beverly Hills over the weekend, he had an ultimatum for Floyd.
"Listen, if Mayweather wants to fight, come fight. You fight in the UFC. We're not boxing him. We did that once. That's over."
Of course, Dana's referring to Mayweather vs. Conor McGregor -- which clocked in at the 2nd highest pay-per-view fight of all time. But, apparently, Dana's not down for a second bite of that apple.
"You wanna fight? You come fight ... a REAL fight."
Mayweather has previously teased that he was training for an MMA fight -- but it doesn't seem like he ever followed through with it.
UFC welterweight champ Tyron Woodley has offered to personally train Floyd and said he truly believes the 41-year-old could have real success in the Octagon.
As for Khabib, White says his next UFC is pretty obvious ... he's gunning for Tony Ferguson, as long as they can overcome a few obstacles.