"Van Nistelrooy is a footballer who has succeeded at all the clubs he has played for. He has scored a pile of goals. He is a forward who can succeed in Madrid," Capello was quoted as saying by Spanish newspaper AS.
Van Nistelrooy's future with United has been in doubt since asking to leave United after a bust-up with coach Alex Ferguson at the end of last season.
Ferguson opted to field Louis Saha alongside Wayne Rooney in the front line during the end of last season, leaving the Dutch striker on the bench.
Real Madrid newly-elected president Ramon Calderon gave the green light to Capello to reinforce his squad in order to end a disappointing three-year spell without silverware.
Meanwhile, Capello said he won't make a move for Arsenal Spanish youngster Cesc Fabregas, despite Calderon's promise that he will sign him.
"Cesc is not going to come into my plans. We are going to try to sign others," the former Juventus coach added.