The German club went down 3-2 at home in the second leg of their last-16 tie to bow out of the competition 4-3 on aggregate, conceding twice in the first half at the Veltins Arena.
"I have no explanation for the first-half performance," he told reporters. "We played like we were doing a couple of weeks ago, without the necessary aggression, and everyone put in maybe 10% less than what was needed.
"We were not proactive and focused enough – we allowed them too much space. In the second half we dealt with them much better. They played with a diamond in midfield and that had surprised us.
"We then coped with the challenge much better. You must not forget that some of our players were sidelined today. The second half was really strong and there is no reason to be depressed now."
Schalke are currently fourth in the Bundesliga.