Izzy, the champion, and Whittaker, the #1 contender, are fighting at UFC 271 in February ... and we asked White if 28-year-old Vettori would be selected as the fighter to step in should 1 of the men be unable to fight.

"It absolutely makes sense," Dana says.

FYI, Adesanya and Vettori fought in June at UFC 263. Israel won by unanimous decision.

Last week, Marvin, still fought -- and beat -- Paulo Costa ... despite the fact the fight took place at 205 lbs. after the Brazilian MMA star showed up fight week and declared he couldn't make weight.

It's likely that most fighters wouldn't have accepted the fight under those conditions ... but Marvin ain't most fighters.

"Listen, he's brilliant to take that fight. First of all, he's a stud. The guy isn't afraid of anybody or any circumstances. He showed up to fight no matter what the weight was gonna be. Didn't let any of that stuff get in his head," White says.

"Stayed cool, calm and composed the entire week. And, what it did for him, it probably made him the #2 ranked guy in the world."

And, Dana was exactly right ... the UFC Athlete Rankings came out Tuesday, and Marvin made a big jump, going up 3 spots all the way to #2.

Costa also fell 3 spots to #6, though, he's still listed as a Middleweight (185 lbs.) ... a division the UFC honcho says Paulo will NOT fight in again.

"[Costa] can't [fight at 185 lbs.]. He's already proven that he can't make the weight. Why would we go through this again? Why would we put him through that again? He's a 205-pound fighter now. We just gotta figure out what the next move is, and who he fights first in that division."

Dana also previewed the UFC's 2 upcoming cards -- UFC 267 and UFC 268 -- both of which are stacked.

267, going down on Fight Island this weekend, features Jan Blachowicz vs. Glover Teixeira, Petr Yan vs. Cory Sandhagen, Islam Makhachev vs. Dan Hooker and the return of Khamzat Chimaev vs. Li Jingliang.