He said: "When he came to Manchester, Ronaldo was a huge talent and today he is one of the best players, if not the best player, in the world. But it's thanks to United that he is the player he is.
"And if Real Madrid really want him, they must be prepared to pay what United really want."
Eriksson warned FIFA president Sepp Blatter that he risked anarchy within football by suggesting that the contracts of players are there merely to be broken.
"There is a lot of money involved for players and for the clubs because football is business today," he added. "There are obviously contracts, but that's not slavery.
"Any work you have, you can't just get up and walk out without paying some sort of fine, or whatever it is. If football really wants to change all the rules, we should all sit down and discuss it.
"But you can't do that overnight. That would be a disaster."