"I leave training and I get the call from [manager Ali Abdelaziz] and I'm immediately excited. That's where you are, you're excited about what could potentially be," 36-year-old Usman told Babcock on the "TMZ Sports" show on Friday (airs weekdays on FS1).

But, the UFC legend said he didn't want "to be hasty and do something unintelligent," so after speaking with his coaches, who didn't object, Kamaru told his people he'd take Tuesday night to decide.

When the sun rose Wednesday morning, the choice was crystal clear for Usman, who "texted Ali and said, 'Let's do this!'"

Of course, Usman is stepping in for Paulo Costa -- the middleweight contender -- who was originally scheduled to fight Chimaev. unfortunately, Costa is dealing with a medical issue, and wasn't going to be cleared.

With less than two weeks before UFC 294, the co-main event was in serious jeopardy ... but then Kamaru saved the day.

Now, Usman faces a huge challenge in the 29-year-old, undefeated Khamzat ... who Kamaru admits is very dangerous.

"He's very skilled. He's strong, he's big. He's fast, he can wrestle, he can grapple, he can strike. He has everything," Kamaru said, "But so do I."

"At the end of the day, we're all human beings. Everyone feels invincible until someone pokes that balloon."

Usman (#1) and Chimaev (#4), are both top 5 ranked welterweights ... but the fight's being contested at middleweight (185 lbs.)

We asked the Nigerian Nightmare if he's been told a win earns him a title shot.

"I mean, it's a no-brainer," Kamaru said, before making it clear he's solely focused on KC.

"I've got a mountain in front of me that I have to scale. We'll get through that first and then we'll worry about what's next."

Check out the full interview ... there's much more with Kamaru, including whether we've seen the last of him at welterweight, where he's arguably the G.O.A.T.