It all started earlier this week ... when 36-year-old McGregor went to X in a now-deleted tweet, writing, "I'd love to crush my left hook into belal's temple and take down the triple crown. I'd do it easily. And fast! I'd cave his skully heavy! The hurdles I face getting back to the octagon are just harrowing."

Conor continued ... "I am so sad over my June 29th fight cancellation. To think this bum is now a UFC champion with zero knockdowns on his resume whatsoever is so bad. The UFC's most abysmal zero revenue generating fighter in modern history."

We asked Belal, who beat Leon Edwards for the 170 lb. title at UFC 304 in July, whether McGregor was a scrap he wanted -- given the trash talk.

"[Conor's] not going to come back. And, if he does come back, I don't think he's ever going to beat anybody," Muhammad said matter of factly, adding ... "He's that old guy that's at the bar. 'I used to be a champion. I used to be double champ, blah, blah, blah.'"

But, BM believes fans will soon come to the realization that CM is washed.

"I think people are going to start realizing that everything [Conor] does is just blowing smoke. He's not really going to fight. He's just talking."

And, if they did ever actually scrap ... Belal is confident it'd be a one-sided affair.

"I would literally slap him around the cage. It would literally be embarrassing to the point of people would be like, 'No way, this guy was a double champion before? How is this guy even a fighter?' The game's passed him up."

We also talked to Muhammad about his other beef with Ilia Topuria AND his upcoming UFC 310 fight against Shavkat Rakhmonov, his first title defense.