United face the Spanish champions at Santiago Bernebeu in the first leg of their Champions League last-16 tie, and the former Red Devils striker believes that Ronaldo is currently the finest footballer on the planet.
"Of course he is the best player in the world," FootballEspana quotes the Norwegian as saying.
"If I could sign one player now it would be Cristiano Ronaldo. I have no doubt. Yes, I know about Messi. He's a fantastic player, four Ballon d'Ors. But if I had to sign a player then it would be Cristiano.
"I played with him for several years when he came to United at only 17. I saw him grow into a man, take responsibility and know how to behave in a dressing room. He is a great guy. I would like a 4-3 [to United], with three goals from Cristiano Ronaldo for Real Madrid."
Solskjaer scored the winning goal for Sir Alex Ferguson's side when they beat Bayern Munich 2-1 to lift the Champions League in 1999.