That's the fight many MMA fans are clamoring for ... believing the new UFC lightweight champ's style matches up favorably with the retired 29-0 superstar from Dagestan.

But, don't prematurely hand Khabib his 1st loss ... 'cause UFC welterweight champ Kamaru Usman -- who's friends with 32-year-old Nurmagomedov -- says he doesn't believe Khabib's any closer to returning to the Octagon after witnessing 31-year-old "Do Bronx's" impressive performance at UFC 262.

"I think Khabib did watch the fight. Was he intrigued? 'Yes, I wanna get back in there.' Probably not," Usman tells us.

"I mean, he's a man that's happy. He's happy with his life. He's happy with the direction that it's going and yeah, I think he's retired."

Of course, Khabib and Dana White met on multiple occasions over the last year ... but the former lightweight champ ultimately decided he was never going to fight again.

So, Charles vs. Khabib isn't happening ... BUT (hypothetically) does Oliveira have the necessary tools to beat the unbeatable fighter?!? Kamaru says no.

"More power to Charles Olivera. I'm impressed by him and I'm a fan for what he's able to do with his career but I just think there's levels to the game and I think Khabib is a different level."

Now, don't get it twisted ... Usman believes CO is a great fighter who went through hell to win the belt.