"If Jake Paul is serious about [MMA] and if he really wants to fight. If he wants to have that respect, he wants to have that attention, he wants people to take him seriously, he's going to have to fight a serious fighter," 30-year-old Lee told us in the aftermath of Jake signing with PFL (Professional Fighters League).

"He's going to have somebody for real, for real. I'm the best option for that. I'm in my prime. I've fought the whose-who of MMA. I've fought six different world champions."

FYI, Kevin's fought everyone from Charles Oliveira, Edson Barboza, Tony Ferguson and Rafael dos Anjos, among a bunch of other top-line guys.

After announcing the signing, 25-year-old Jake, 6-0 as a professional boxer, issued a challenge to Diaz -- a boxing match followed by an MMA fight -- against the combat sports superstar from Stockton, CA.

It's not that Kevin doesn't like Jake vs. Nate ... he's actually into the fight.

"Nate Diaz is a good fight. I'll give him that, but there's two problems with the Nate Diaz fight. Nate Diaz ain't going to take that fight, let's be honest. They're going to go back and forth for months and years," Kevin said, explaining Diaz would first need to sign with PFL, which Lee says will not happen.

Kevin says if they make it happen, that's great for them ... but if Jake wants to fight a top-of-the-line MMA fighter in the near future, Lee's the guy.

"If he makes the Nate Diaz fight happen, great but I don't see that happening within the next year or so and if he wants to fight this year then he should fight somebody that's on the roster and already making it happen. Nate Diaz is a good fight, though. I ain't hating."

We asked Lee, who says he's on the verge of signing with PFL, whether he had a message for Paul.

"Let's make it happen. Be a man. Put your money where your mouth is. Step in there. You might lose. You're going to lose. It's going to happen but I'm gonna shake your hand afterward and let's make some money."