Khabib's Manager Shoots Down MMA Fight With Mayweather, 'It'd Be Manslaughter!'

Khabib's manager tells TMZ Sports if a fight with Floyd Mayweather goes down ... it's gotta be in the boxing ring -- 'cause the UFC star would end up behind bars if the two fought MMA!!

"An MMA fight, I don't think any state athletic commission should allow that," Ali Abdelaziz tells us ... "It would be illegal. It would be voluntary manslaughter."

Of course ... a fight between Nurmagomedov and Floyd has been rumored for weeks -- with Ali telling us he's actually had conversations with Money and his team about a possible tilt.

But, Dana White told us earlier this week, if the two do square up -- it's gotta be MMA ... and that's something Ali says is a horrible idea for BOTH Floyd and Khabib.

Instead ... Ali seems much more open to the idea of a boxing match -- and says Khabib could easily win that too -- without going to jail for murder.

"Khabib is wide. When Khabib is boxing him, he will cover him. He will put his body on him, make him tired. Carry the weight and get him tired. I think 100 percent [Khabib can win]."

The problem? Khabib's manager tells us he thinks Floyd wants no part of the match -- saying Mayweather's all talk.

"I think Floyd just wants to keep his name relevant. He's 57 years old. He's almost 60 ... He can't do nothing to Khabib."

Still ... Khabib clearly wants it -- writing on his Instagram Friday, "Where are you? @floydmayweather #27vs50."