UFC star Kevin Lee says Conor McGregor's gonna have to go coffin shopping if they ever square off ... 'cause he's comin' for the Notorious' belt after UFC 216 ... and says it ain't gonna be pretty.

TMZ Sports spoke with Lee about his plans after taking on Tony Ferguson for the interim lightweight title -- which he sees as the hardest fight in the division, not Conor.

The Motown Phenom says he's been doing all of Conor's hard work -- which makes him the REAL champ -- and he's not afraid to take on McGregor.

Lee makes his point clear by saying if he were to cross paths with Conor in the gym, he'd be in serious trouble:

"I'ma f**k him up, you feel me? ... He knows if the folks wouldn't break it up, I'll kill the man. I'll murder him."