Holtby, 23, recently joined the Bundesliga club from Tottenham Hotspur until the end of the current campaign.
The German international's deal also includes the option to make the move permanent next summer.
"Lewis Holtby is a really good player. We need players like him," Van der Vaart is quoted as saying by Sky Sports News. "He is technically good but also good at tackling. I feel that Hotlby will be an important player for Hamburg this season."
Holtby joined Tottenham from Schalke 04 in January 2013.