Rafael spent one season playing alongside Ronaldo at Old Trafford before the Portuguese winger moved to Real Madrid in 2009.
The Brazilian full-back will come up against his former teammate at the Bernabeu on Wednesday but believes that Ronaldo will offer nothing he has not seen before at close quarters.
He told reporters: "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's good but I don't know what he can do'. I know what he can do. If you concentrate, that's the key, reading the game. For me, he's the best player in the world.
"I think he's the complete player and I really like him. I think he's the number one. I'm sure he'll get a good reception back here [at Old Trafford]. I think it'll be massive."
The 22-year-old has enjoyed a breakthrough season at Old Trafford, establishing himself as a first-choice defender under Sir Alex Ferguson.