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 ... he has no boxing skills."

Buster says Conor's only shot is hitting Floyd with a "Hail Mary punch" -- but he expects Floyd to flat-out dominate. 

FYI, Douglas was a 42-to-1 underdog when he knocked Tyson out. 

Right now, Conor's about a 4-to-1 dog ... but over 90 PERCENT of bets are being put on him to upset.