Wednesday's 2-0 defeat was Dortmund's first loss in Europe this season and added to their frustrating form in the Bundesliga.
Klopp's side have only won three times all season domestically and sit third bottom in the table with only 11 points from 12 matches.
However, the head coach believes things are beginning to come together and Dortmund are ready to turn their campaign around, starting at Eintracht Frankfurt this weekend.
"This is no picnic," he said of Dortmund's current situation. "You have to deal critically with it.
"The team is now fitter than at the beginning of the season. We must not lose the positive views of the guys.
"None of the players is at the upper limit, we have to keep working. All of our attackers weren't good enough in London, but the key is to keep staying positive.
"We are convinced that we will fix this situation. We must all take responsibility."
Klopp will make late checks on the fitness of Mats Hummels (ankle) and Sven Bender (hip) before Sunday's clash, but Jakub Blaszczykowski (muscular injury) is still away off a first-team return.