The ex-Germany defender has made it clear that it is part of the club's philosophy to bring in big names, regardless of the squad's quality but believes the superstar is immune
The Blancos replaced accomplished players Angel Di Maria and Xabi Alonso with James Rodriguez and Toni Kroos this summer and Metzelder has stressed that it's part of Madrid's marketing strategy to always try to bring in the biggest names around.
But the 33-year-old feels Ronaldo is indispendsable to the Spanish giants, while the remainder of the squad are "interchangeable."
"Its part of Madrid's strategy to always sign new stars. They have a feeling that they cannot afford to have a summer without a big transfer," the German told Kicker.
"They are the best team in Europe as Champions League winners, yet they keep bringing in new players for crucial positions. Cristiano Ronaldo is one of the few untouchables, plus some of the Spain internationals. All of the others are interchangeable.
"When a player makes a name for himself at a tournament, that immediately makes him an interesting option for Madrid. Real Madrid are a global brand and it's about building your brand as much as it is about football.
"Angel Di Maria did not do enough for Madrid's brand. But if you'd asked his team-mates, they would probably all have said that Madrid should have held on to him. It was the same for Mesut Ozil last year.
"It would have been better for Kroos had Alonso stayed. Now people will start saying Alonso would have done better if Kroos disappoints. Kroos will have to get used to the fact that Madrid prefer defensive stability over entertaining fans."
Di Maria eventually left Madrid for Manchester United, while Alonso departed the Santiago Bernabeu side for Bayern Munich this summer.