Logan Paul's a "fake fighter" who has no business being in the ring with Floyd Mayweather ... so says his own brother, Jake, who tells TMZ Sports the dude is "f***ked" against the 50-0 boxer.
Of course, the YouTube superstar is set to face off against "The Best Ever" in a boxing exhibition on Feb. 20 ... a fight that has been met with mixed reactions.
Jake -- who's fresh off an insane KO of ex-NBA star Nate Robinson -- thinks Logan is pretty much doomed against Floyd .