TMZ Sports ran into the 38-year-old (retired, for now, at least) former UFC champion at LAX on Friday ... and asked him about the Problem Child's foray into MMA.
"Stop trying to fight little dudes. F***ing  pounders. You're a middleweight. If you want to fight a middleweight, I got hands. We can do anything we want. I really don't care. I'd beat that guy in my sleep."
As we previously reported, 26-year-old Paul recently signed a multi-fight, multi-year deal with PFL ... and wasted no time by calling out Nate Diaz for a boxing and MMA fight.
Diaz hasn't yet accepted the challenge ... and if he doesn't, Rockhold is more than ready to step in.
"I got some freedom to do some things and as you can see, I like throwing hands these days," Luke said, adding, "fight people your own f***ing weight. Straight up. And champions, a real champion, a real contender. A real someone who can put you on your ass. not old guys, 55-year-olds, or lightweights. You can't even get above a welterweight, dog, So, step up."