The result saw Dortmund fall six points behind Leverkusen in the Bundesliga table and 10 behind league leaders and reigning champions Bayern Munich.
"If you want to beat Leverkusen then you've really got to be on top of your game - which we weren't today," Klopp told the official Bundesliga website.
"We started well, closed off the space and defended high, but they forced us into too many long balls and we became too passive.
"In the second half, we were much more passionate, but our performance wasn't as good as it could have been."
Dortmund will look to get back to winning ways when they travel to face Hoffenheim on Saturday before going into the winter break with a home clash against Hertha Berlin a week later.