UFC's Luke Rockhold -- 'I Wanna Slap the S**t Outta Conor McGregor'

Conor McGregor is no Ronda Rousey ... he's not even a champ ... and he's got a long way to go before he'll ever be considered the "best" ... so says UFC stud Luke Rockhold.

Luke was at The Palm restaurant in L.A. -- hangin' with fellow UFC superstar Urijah Faber -- when we brought up McGregor ... who's notorious for bragging about how great he thinks he is.

Enter Luke -- who says the guy needs to come back down to Earth.

"He's a lot smaller than he says he is ... literally and figuratively."

Luke says he thinks Conor's talented -- and great for the sport -- but he's unproven and has major holes in his game.

"I'm lying if I'm saying I'm not entertained by him at some points, but other times I wanna slap the sh*t out of him."

As for Urijah, he likes Conor and thinks he's a beast -- but his issue has more to do with McGregor's fiscal irresponsibility.

Check out the clip -- GREAT financial advice from Faber.