Dortmund occupy 10th spot in the Bundesliga after losing 3-1 at third-placed Borussia Monchengladbach on Saturday, with goals from Oscar Wendt, Raffael and Havard Nordtveit sealing victory for Lucien Favre's side.
And Klopp stressed that Dortmund's next two fixtures at home to Paderborn and Eintracht Frankfurt will prove crucial if they are to retain any hope of qualifying for the UEFA Europa League.
"The season has been, and remains, difficult," said the Dortmund coach.
"Now there are two enormously important games for us, because we should score points because we are at home.
"If we do not win, chances of Europe are gone."
Dortmund got off to a terrible start at Borussia-Park, Wendt giving the hosts the lead after just 28 seconds.
Rising Monchengladbach star Patrick Herrmann caused havoc for Dortmund's floundering back line throughout, and the winger set up the second with a surging run from deep, teeing up Raffael to slot into an empty net.
Nordtveit then added a third shortly after half time, with Shinji Kagawa having squandered two golden opportunities just moments earlier.
Ilkay Gundogan did pull a consolation goal back for the visitors, but Klopp conceded that his team were outplayed by the UEFA Champions League hopefuls.
"A mixture of good will and high readiness from Monchengladbach, and lack of consistency from us are to blame," he added.
"Every goal was so easy, the opposition has made the most of our errors.
"We get into duels up the pitch and then the opponents win the ball and turn towards goal, it happens time and time again.
"Our performance was no relapse to how we played earlier in the season, but still obviously very modest."
