Sahin arrived at Real Madrid in the summer as one of the brightest prospects in the world after being voted as Bundesliga's Player of the Season but has since made only one appearance as a substitute in La Liga for the Spanish giants.
Metzelder had a similiar fate when he played for Real Madrid between 2007 and 2010, where he made only 23 league appearances during an injury-hit spell.
The current Schalke defender warned that it will be difficult to break into Los Blancos' first team.
Speaking to AS, Metzelder advised Sahin: "If you arrive and don't take off immediately, no-one will wait for you at this club.
"Real Madrid win nine out of 10 games, so you need to be successful from the beginning. I think it will be difficult for him."
Metzelder also added that the competitiveness in the star-studded team can be also as crucial as injuries in deciding who gets a spot in the first XI.
He continued: "My lack of continuity was not only down to injuries, but the competition was so great. There were situations where I felt every confidence from the club, and some when I did not.
"To succeed in Madrid, you have to be more than just a good player."