Jurgen Klopp's outfit has slipped to third in the table, 12 points adrift of leader and reigning champion Bayern Munich, after suffering a third home defeat in a row Saturday.
Marco Reus gave BVB the lead seven minutes in but Hertha Berlin scored twice before the break to condemn the Champions League runners-up to another defeat.
"The defeat is very, very disappointing - it's our third home defeat in a row and we're going to have to wait until the start of next year to put things right," said Kehl. "We stopped playing football after making a good start to the match. We let Hertha back into the game with those goals and we couldn't break them down in the second half."
Midfield partner Nuri Sahin echoed Kehl's sentiments, urging the team to be more ruthless.
"We should have added to our lead after going in front, but we took our foot off the gas," he said. "We need more of a killer instinct to win these types of matches. Had we scored the second goal, Hertha wouldn't have got back into the match. They defended well in the second half and took advantage of our errors in the first. We're very frustrated."