UFC star Israel Adesanya is a phony champ who hasn't beaten any level fighters ... so says Paulo Costa, the dude who just beat Yoel Romero.
TMZ Sports talked to Costa -- the #2 ranked UFC middleweight -- about his next move after his epic win over Romero ... and it's clear he ain't impressed with the 2 champs in his division.
"Yoel Romero is more tough than Whittaker, I know. And, man, Adesanya, Adesanya is a fake champion."
Why does Costa -- the 28-year-old undefeated (13-0) fighter -- think Izzy's a fake champ???
"Because he not proven yet. He needs to prove. He won against who? Tavares? Derek Brunson. Gastelum. Gastelum is good fighter, but Gastelum is not a middleweight. Against Anderson Silva. Man, Anderson Silva is not top 10. Not top 5. I beat a real beast. I beat Yoel Romero."
FYI -- Adesanya won the interim belt against Kelvin Gastelum earlier this year.
Whittaker (middleweight champ) and Adesanya (interim middleweight champ) are fighting October 5 at UFC 243 in Australia ... Costa says he'll be there, and he has a message for both fighters.
"I am a real deal. I'm training like a crazy guy. This is my life. [I'm] born to fight. [I'm] born to win, and I will take the belt."