"In these types of things it is best to be realistic. Cristiano Ronaldo right now is a 10 out of 10, the best in the world," said Schuster.
That does not mean Ronaldo, who took his tally to 36 goals this season during Tuesday's Champions League triumph over Roma, will be off-limits in the future though, according to Schuster.
"In the short term I cannot see it as being possible, but I think in a few years that he could wear the Madrid shirt in the Bernabeu," Schuster told AS.
"He is the star at Old Trafford and until he wins a Champions League and some more Premier League titles they are not going to allow him to escape.
"For me, Cristiano Ronaldo is already better than Kaka after the Brazilian's loss of form at Milan this season. Some day he will be with us..."