Mike Tyson says if the UFC superstar was his next opponent in his return to the boxing ring ... he'd beat the hell out of The Notorious -- saying, "I'mma kick his ass!"

Iron Mike went on 'The Tonight Show Starring Jimmy Fallon' on Monday to promote his new huge comeback fight with RJJ ... when Jimmy decided to play a little "Who Would Win?" game.

Rocky? Mike says he'd dominate. Ivan Drago? Tyson says he'd beat that ass too. Drago's too bulky and too damn slow.

As for McGregor? Mike says that'd be no issue ... as long as they played by boxing rules!!

The clip is funny ... Tyson says there IS one opponent who'd beat him -- the 'Karate Kid' -- saying, "I'mma have to surrender to him on this one."

By the way, a fight between Conor and Mike would almost certainly never go down ... the age gap (54 to 32) is huge, and the two are great friends these days. They even smoke weed together!