Schalke can leapfrog Jurgen Klopp's side into second place in the Bundesliga with a victory at Signal Iduna Park, where they were 2-1 winners last season.
The Gelsenkirchen club beat Eintracht Braunschweig 3-1 on Saturday to extend their winning run in the league to three games and Keller has urged his side not to get carried away by the occasion when they face Dortmund.
"It's the game that everyone looks forward to," Keller told Schalke's official website.
"But we have to keep a cool head."
Keller also called for his players to seize the moment, but added that Schalke's season will not hinge on the result of Tuesday's eagerly awaited derby clash.
"When you have the opportunity, you want to take it, but the important thing is where you are after 34 matches have been played. Our objective remains a place in the Champions League.
"They're at home and when I see the team our opponents put out despite all the injuries it's to their credit."
