The biggest fight in UFC history will end in a loss for Conor McGregor -- 'cause he's NEVER fought a dude with strength like Khabib ... so says UFC star Kelvin Gastelum.
Kelvin tells TMZ Sports he's heard all about Khabib from other fighters ... and he sees no way McGregor will be able to overcome his power when they square off at UFC 229.
"Every time somebody speaks about Khabib, they tell me how strong he is and how he feels like a heavyweight once he gets a hold of you," Gastelum says.
"So, I feel like if he grabs a hold of Conor, he's never felt that kind of pressure and I think Khabib will get it done."
Kelvin's bold fight predictions didn't stop there ... 'cause he also tells us he's going to win his upcoming title bout with Robert Whittaker -- and he's even giving us the round it'll end in!!
By the way ... there's still no date set for that fight while Robert recovers from a broken hand.
BUT ... in the meantime, you can watch the two talk trash to each other all season as coaches on "The Ultimate Fighter" -- which debuts Wednesday on FS1.