Khabib Nurmagomedov is staying busy in retirement -- campaigning to be Luke Rockhold's fight coach for Luke's UFC return ... and it sounds like it could really happen.

"He’s assuming this coaching role now," Rockhold tells TMZ Sports ... "He wants to be a big part of my comeback."

Luke and Khabib have been training together for years -- but now that Khabib is done fighting, he wants to transition into coaching ... and take Rockhold on as his client.

Rockhold hasn't fought since July 2019 -- when the former middleweight champ made his light heavyweight debut against Jan Błachowicz.

The 36-year-old tells us he's been dealing with a shoulder issue that's kept him from taking a fight -- but he's recovering well and should be good to go in "late Summer."

Rockhold says he hasn't made an official decision regarding Khabib's offer ... but Luke is obviously considering the move.

In the meantime, Luke says he's heading up to Vegas to train with Nurmagomedov for a week -- and he'll probably make his mind up after that.

Either way, Luke says he's convinced Khabib will be just as good of a coach as he was a fighter.

"He’s going for the 'Coach of the Year.' That’s what he wants. He wants to be the next great thing, following in his dad’s footsteps."

"That’s what his dad always did. His dad was always insightful, and always had a good eye for the sport ... and I think Khabib’s ready to assume that role in fatherhood."

As for Luke's career. Rockhold says he's likely done fighting at light heavyweight and will resume his career back at middleweight.

"I believe [185 pounds] is my calling. I’m about 205 pounds right now and I’m just naturally there."

Luke adds, "It wasn’t really me to bulk up. I was lifting weights and did my thing to get up to 205 and I could barely hold 215, 220 max."

"I feel good. This is my championship weight. This is where I’ve done all my damage and I like the route to victory of what’s ahead of me, so I’m ready to go."