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.