Volkanovski joined Michael Babcock on the "TMZ Sports" TV show (airs daily on FS1) on Wednesday ... where Alex "The Great" talked about the phone call he received late last week from his manager.

"I think I might've had Reign, my newest addition to my family, my newborn Reign," Alex said ... "I might've been holding her, and [my manager] gets on the phone and says, 'Mate, you wouldn't believe it.' And I'm like, 'What's up?' Did not cross my mind one bit. I'm thinking it's for something else. He goes, 'It's looking like Charles isn't going to make it and they need someone.'"

Volkanovski was obviously shocked ... and after conferring with his coach, Joe Lopez, and wife Emma, the decision was clear -- we've got a fight in less than two weeks, and it's going down halfway across the world.

Of course, Makhachev was originally scheduled to fight Oliveira -- a rematch of UFC 280 (Islam won by 2nd round submission).

In some ways, Volkanovski, who lost by unanimous decision to Islam in February (although many people think he actually won), believes the lack of a training camp could actually be beneficial.

"I'm feeling good. I'm excited. That's one thing that I've taken out of all this whole thing, obviously, it's crazy. But man, it's gonna be fun. I don't think I've ever been this excited for a fight.

"Obviously you get excited, but you go through camp, and camp can be exhausting. Mentally and physically exhausting thinking about the fight the whole time. Now I don't have time for that. As soon as I heard about it I had a smile on my face and it hasn't gone away even until now."