Kamaru says it would "absolutely" be the biggest fight in UFC history -- but the champ isn't holding his breath that it'll happen.

Remember, Khabib (29-0) announced his retirement back in October 2020 after defeating Justin Gaethje -- but later told Dana White he would consider a comeback if one of the top fighters showed him something "spectacular."

Well, enter Usman ... who's been destroying the best fighters in the UFC's welterweight division including wins over Jorge Masvidal, Colby Covington, Tyron Woodley, Gilbert Burns and more.

The question -- is Khabib impressed enough to consider a comeback against the reigning welterweight champ?! Usman isn't counting on it.

"I’ve lost faith in this one," Usman tells TMZ Sports ... "I think you have more of a chance at making Kamaru Usman and Georges St Pierre happen before Kamaru Usman and Khabib."

We also asked about a possible matchup with Nick Diaz down the line ... and Usman seems interested!