The club captain pointed to the number of players still working their way to full fitness as the reason behind the surprise defeat on Tuesday.
A first half strike from Henrikh Mkhitaryan opened the scoring for BVB, before Pierre-Emerick Aubameyang sealed the victory just afer the hour mark.
Bayern looked off the pace throughout, with five members of the squad part of Germany's successful World Cup campaign and still in the early stages of pre-season.
Lahm - who started from the bench before replacing Thomas Muller at half-time - was quick to point to that fact and said it is simply a case of the side getting more matches under their belt.
He said: "We have a lot of players that have been in training only for a short period. We had a lot of injured players that stayed home. That's why it was hard.
"We are in the middle if the pre-season. In training, you can only lay the ground, but not simulate the competition."
One cause for concern will be the injury suffered by Javi Martinez, who was withdrawn after just 31 minutes with what appeared to be a serious knee injury.
Bayern open their Bundesliga campaign against Wolfsburg on August 22.