Dortmund, last year's Champions League finalist, lost 2-1 at home to VfB Stuttgart in the Bundesliga on Saturday in coach Niko Kovac's debut, to drop to 11th place.
They have won just one of their last six league matches but have reached the Champions League playoffs for a spot in the competition's Round of 16.
"It is not easy (to turn things around)," Can told a press conference. "Not everything in the past weeks has been negative.
"We know it is about winning but you have to try to stay positive. The negative will not take you further.
"The Champions League is also different to the Bundesliga," he said.
"We had a good run last year. It is somewhere where we can be successful again."
Dortmund lost 2-0 to Real Madrid in last year's Champions League final but their recent Bundesliga performances have put next season's participation in Europe's premier club competition at risk.
"No one believed it last season and we reached the final," Can said. "I don't want to talk about the final but we can try to be successful."
Kovac will be without Ramy Bensebaini, who picked up a muscle injury against Stuttgart but is expected to be fit for the league on the weekend.
"We are facing a tough team," Kovac said. "To be first in Portugal is very difficult. That means the quality is very high.
"This is a football-playing team. From the back to the front. This means for us we must be aware of the technical skills and focused over 90 minutes.
"This is the first part of two games. We want to get a good result to have a good opportunity to reach the next round."