The Germany international was not impressed by his side's defending, while Manuel Neuer was equally disappointed with the Bavarians' defeat.
Stefan Kiessling gifted Leverkusen the lead in the opening 45 minutes, before Mario Mandzukic leveled the scoring in the 77th minute.
Nevertheless, Sidney Sam eventually helped the Werkself to three points with his goal in the closing minutes of the game, much to the dissatisfaction of Lahm.
"Leverkusen's victory isn't undeserved, because they scored two goals," Lahm was quoted as saying on the official Bayern website. "We let in a ridiculous and stupid opening goal, we fought back and made it 1-1, and then we let in an equally dumb second goal."
Manuel Neuer was also disappointed with Bayern's defeat, but was adamant that the loss had nothing to do with the club's attitude.
"We should perhaps criticize ourselves for not taking the lead. At the start, Leverkusen came here and tried to play football, but we should have scored the first goal," he said. "Instead we were forced to come from behind. We gave it everything we had in the second half and tried to salvage something. The defeat wasn't due to our attitude."
Bayern currently holds a four-point lead over second-placed Schalke.
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