The 22-year-old right-back has praised his former team-mate ahead of their Champions League clash on Wednesday, and believes the Real Madrid forward is better than Lionel Messi
The Portugal forward has 36 goals in 35 appearances for los Blancos this term, and has netted a sensational 182 in 179 games since his €96 million move from the Red Devils in 2009.
Despite Ronaldo’s scintillating form in front of goal for the Liga giants, his former team-mate says his time spent training against the 28-year-old will give him a vital edge on Wednesday night.
"For me, [Ronaldo is] the best player in the world," he told The Sun. "I think he is the complete player and I really like him. I think he is the number one.
"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."
With Rafael performing well against Gareth Bale during United's recent 1-1 draw with Tottenham at White Hart Lane, the defender suggested concentration is key to keeping out top-class players in big games.
"If you concentrate, that’s the key, reading the game," he continued. "[Ronaldo and Bale] are 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."
Sir Alex Ferguson has sung the full-back's praises of late, and suggests competition for places is driving the side on to succeed this season after increasing their lead at the top of the Premier League to 12 points.
"I like it when I hear Sir Alex Ferguson or Rio [Ferdinand] saying these things about me," he added. "It's nice and I hope I can keep going. It's important to get the team medals first of all because, if you get them, then the other personal things are going to come as well.