UFC's Urijah Faber -- Conor Got Whooped ... I Tried to Warn Him!

UFC superstar Urijah Faber says he TRIED to help Conor McGregor prepare for Nate Diaz -- but Conor ignored him -- and the result? "HE GOT WHOOPED."

We spoke with Faber at the UFC Gym in La Mirada where he confirmed that UFC honcho Dana White offered him the McGregor fight at UFC 196 (and Faber accepted) ... only for Conor to turn him down to fight Diaz instead.

"That backfired!" Faber told TMZ Sports.

Despite the rejection, Faber -- who has the most submissions in the UFC Bantamweight division -- says he tried to help Conor by giving him tips on how to fight the bigger, stronger Diaz ... but his advice fell upon deaf ears.