The Portuguese international has long been linked with a switch to the reigning la Liga champions, with United chief Sir Alex Ferguson growing increasingly irate with the Madridistas' advances, once commenting that he 'would not sell them a virus'.
Further speculation involving Ronaldo and Madrid is expected this summer, with several contenders for the Real Madrid presidency sure to promise the FIFA World Player of the Year's signature in exchange for votes.
However, Spanish stopper Casillas believes Real should forget attempting to lure stellar names to the capital, and instead target home-grown talent.
"Ronaldo and Kaka are great players but we don't have to keep returning to these names," he said.
"What we need is a team that compete with Barcelona and one player doesn't change a team. I believe a team always forms together and there cannot be a player that changes everything.
"If we pay €100million for Ronaldo it's because he's worth it. But with the economic crisis it is peculiar that in football we talk of these astronomical figures.
"The Spanish national team are great and my team-mates at international level are fantastic, that is clear.
"I have always believed in the Spanish player and right now they are playing great football."
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