كتب : وكالات
"We have a lot of young players who have to try to turn their talent into performances on a regular basis," the Dutch manager told Hamburg's official website.
"Our midfielders could score more goals. If they can do that we will have more goals to our name, but you cannot expect us to score 30 more without van der Vaart."
Van der Vaart penned a five-year contract with Real, for a fee believed to be around 15 million euros.
The 25-year-old's departure could help Egyptian forward Mohamed Zidan gain more first-team football next season.
Zidan put in a fine display in Hamburg's 2-1 defeat to Real in a friendly game, scoring a spectacular goal against Los Merengues.