The attacking midfielder will start the 2013-14 campaign at the Bundesliga champions as Bayern triggered his 37 million euro buyout clause.
Hummels is not amused with his former teammate's decision to join BVB's domestic rivals and hinted that money was the overriding motivation behind the move away from Signal Iduna Park.
"I do not understand Gotze's decision and I have told him just that. Everybody at Dortmund was surprised. This only further shows that this was never an issue," Hummels told Sport Bild. "Everybody can see how good our team has become. I simply do not believe that there were any sporting reasons to leave BVB.
"It was clear to see that the team had really made steps forward and Mario got along with all of us very well. That's why it annoyed me so much that he felt it was necessary to leave at such an early stage of his career."
Gotze, 21, is a product of the Dortmund youth academy and played a key role in its Bundesliga titles in 2011 and 2012.