"It's a pity that we didn't achieve our goal, which was to beat Dortmund, but quite frankly, I have to admit that the end result was fair," said the 67-year-old.
"Our opponents once again proved they are a very organised team who are well prepared physically. They played aggressively and tried to attack whenever possible, creating a lot of problems for us. We couldn't keep the lead we had twice established and we made some errors. That is inexcusable.
"We made schoolboy mistakes today and now we have to work hard. We talked a lot about set pieces while we were preparing for Dortmund, but they still used one of their big strengths to score."
The second leg takes place at the Westfalenstadion in two weeks' time.