The former Barcelona goalkeeper is unsure whether the youngster is up to the task of replacing the outgoing Spaniard if a move goes through this summer
Valdes will depart Camp Nou when his contract expires in June and the Blaugrana have confirmed their interest in the young German after he turned down the offer of a new deal with his current employers.
But former Barca keeper Rustu fears the move could come too soon for the 21-year-old and says very few German stoppers are successful in La Liga.
He told The Network Radio: "Ter Stegen is not at the level of Valdes. Besides none of the German goalkeepers have been successful in Spain.
"I met with Valdes when he was very young. At the time he seemed a bit crazy but he has matured into a very good and experienced goalkeeper. I would advise him to continue at Barca."
Rustu then gave his thoughts on the attributes required to be a success between the posts for the current Barcelona side.
"You need to be good with the ball at your feet and have good technique on the ball."