Conor McGregor beating Floyd Mayweather would be the biggest upset EVER ... this according to the man responsible for the most shocking upset in boxing history.
We talked to Buster Douglas with MayMac about to go down ... and he told us that Conor taking out Floyd would be waaay crazier than him KO'ing Mike Tyson back in 1990.
"I think his odds are a lot more tougher than mine," Buster told TMZ Sports.
"Conor is no boxer .