... actually, that might be too generous of a comparison.

Roy gave an official 0.0% probability of a Logan Paul victory -- and when he breaks it down, it's clear RJJ thinks Floyd is gonna straight-up embarrass the YouTube star.

"You're talking about the greatest defensive fighter of all time," Roy explained to TMZ Sports ... "so, it's very hard to hit him."

"That being said, I don't give the kid a chance."

We pushed back and asked about the famous "puncher's chance" theory -- that anyone with power can end a fight if he can land one shot.

Roy wasn't having that AT ALL.

"C'mon bro, that's Floyd Mayweather. Are you serious?"

"If he was Mike Tyson's size, it'd be different. He's not Mike Tyson's size ... it's not gonna happen."

Oh, there's more ... Roy explains in detail why 44-year-old Floyd is gonna light up the 26-year-old when they clash on June 6.

Meanwhile, Logan says he LOVES the doubters -- and can't wait to prove everyone wrong.