Rafael also reckons Ronaldo is fallible. And he insists he has the ability to outwit him having faced him in training in his formative years at United.
Rafael, who starred in the 2-0 win over Everton on Sunday, said: "I know what he can do because I played and trained with him a lot.
"So he's not the type of opponent where you would say 'He is good but I don't know what he can do' - I know what he can do."
Having recently kept in-form winger Gareth Bale quiet in United's draw against Tottenham, Rafael is well prepared for what Ronaldo can throw at him.
He said: "If you concentrate, that's the key, reading the game.
"They're both very quick but maybe they are just different. Ronaldo is right-footed and plays a different type of game.
"He can come inside and shoot with power but both of them are tough players to come up against."