Sylvester Stallone says he'd definitely pay good money to see Floyd Mayweather fight Conor McGregor -- though he's pretty sure Floyd would beat that ass!
"In boxing, you gotta go with the boxer," Sly told us ... but noted "anything is possible" if Conor can land one good clean shot.
As we previously reported, Floyd's trying like hell to get a deal done ... but we're told it's nowhere near close at this point.
