Bayer won 2-0 at the Etihad thanks to goals from Alejandro Grimaldo and Patrik Schick.
Speaking to City Studios after the game, Reijnders said: "Unfortunately we didn’t play very well, although we created pretty much some chances as well. But we could have done much better. Getting two goals against us is hard of course.
“We really trained on the tactics. Of course, Leverkusen also played pretty well but two times that they had a shot on target and two times it was a goal. We had to be more secure with our chances.
“Today was a day that it didn’t want to go in. But we have to keep confidence and be ready for the next game on Saturday.
“Of course this is a hard pill to swallow but we have to keep the confidence because we showed in a lot of games what we can do and get back on that winning track.”