One year after Floyd and Conor faced off in the boxing ring, TBE says he learned firsthand McGregor is a "warrior" who's not afraid of anything ... and he's passing on a warning to his UFC 229 opponent.

"Conor's not gonna back down and he's not a scared fighter at all, win, lose or draw," Mayweather said at the Hard Rock Hotel and Casino in Atlantic City ... "Conor gonna come to fight."

Despite their rivalry, Floyd says he actually wants to HELP Conor this time around ... saying he'd love for CM to train at his home gym in Vegas.

"I know that UFC got a training facility but we'd like for him to work out at the Mayweather Boxing Club."

So, who's Floyd picking to win the fight?

"I don't really have a prediction because I've never seen the guy fight that's facing Conor McGregor -- from what I know he's a hell of a guy that's on the ground -- hell of a grappler -- and he can wrestle."

"But, the fans want to see you stand up and fight. That's what the fans like to see. But, Conor McGregor, he's a tough competitor, like I said before Conor McGregor's not gonna back down from anyone, he's not scared, he's a warrior."