Die Knappen, who have slipped to ninth in the Bundesliga with just one win since November, left Turkey leading their Champions League tie on away goals thanks to Jermaine Jones's first-half strike at the Turk Telekom Arena.
Keller believes that this performance, coupled with his team twice coming back to avoid defeat against Mainz, has given their season renewed impetus.
"I hope we take the momentum from the Champions League into the game against Fortuna," Keller told reporters.
"I expect [Dusseldorf] to play aggressively and fight for every ball. We are aware they've picked up some points in the league so far and we know it won't be easy.
"We took two important steps in Mainz and Galatasaray, but we need to progress from there. We haven't achieved anything yet."
Keller took over from Huub Stevens in December.