Dortmund took command of the Bundesliga title race, moving seven points clear of their rivals as goals from Marco Reus, who got two, and Paco Alcacer outweighed a couple from Robert Lewandowski, who had twice put his side into the lead.
Hummels, who was suffering from a cold, felt culpable for his part in the loss, giving the ball away cheaply in dangerous areas.
“Unfortunately it got me a bit on Friday. I really wanted to try playing, first because it’s such a big game and secondly for myself personally,” the former BVB defender admitted.
“But there you have it, you saw my head was not quite there. Two such mistakes since 2010 or around then.”
The centre-back felt his side turned in a decent display overall.
“We played a very, very good first half, completely dominated the game and deservedly got to the break with the lead,” he said. “Then it became more open in the second half. We had done well to prevent them countering in the first half, their main strength, but that came in the second. We didn’t help ourselves by losing the ball easily, and I was partly to blame.”
The next few weeks will be vital in the title race.
“There are still six games left until the winter break,” he noted. “If we make it up a bit before then, it doesn’t look so bad, but if we lose a few points before then, then that’s that. So, we have to make sure we can get the job done, build on our first half, and get back to our real game in the next few weeks.”
After the international break, Bayern will play host to Fortuna.