The northern Germany club's recent revival was brought to an end by Stuttgart last weekend, when a win would have taken them up to fourth in the table.
They now travel to Augsburg knowing they need to bounce straight back or else they could find themselves dropping down the table quickly.
"If you think that we play at home to Bayern Munich next weekend, then tomorrow night [Friday] is immensely important," said Van der Vaart.
"We've got to win three points again in Augsburg. We must not let our heads drop and we must concentrate on getting these points."
Defeat to Stuttgart has not damaged morale though, according to the Dutchman.
"It happens to any team," he said. "It will not knock us down. We all remain optimistic.
"When you are right down at the bottom for three games and then you go four games unbeaten, then you can have a bad day once in a while."
Augsburg have had a few too many bad days this term, but a draw with Bavarian rivals Nurnberg last season has kept them above the bottom two.
Despite picking up six points in their last six games, coach Markus Weinzierl has announced he will be changing his goalkeeper.
Regular number one Simon Jentzsch has missed those six games with a finger injury, but he will return and replace Mohamed Amsif on Friday night.
"Mo has done a really good job, but Simon Jentzsch is our number one," said Weinzierl. "Therefore, he will be in goal against Hamburg."
Weinzierl may also have to make a change in midfield with Daniel Baier struggling with a muscular problem, while the visitors are without Petr Jiracek, whose place will be taken by Maximilian Beister.