Hummels sparked considerable anger amongst Dortmund's fanbase on Tuesday, as his summer transfer to Bavaria was confirmed, along with Bayern's capture of Benfica midfielder Renato Sanches.
Hummels began his career at Bayern before departing for Dortmund, initially on loan, in 2008, and has won two Bundesliga titles with the club, as well as reaching the Champions League final in 2012-13.
Having captained the team since 2014 Hummels' move to Bayern came as something of a shock in Dortmund, but he maintains that fans will soon come to understand his reasoning.
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"Maybe soon people will understand why I have made this decision," he told reporters after Dortmund's 2-2 draw with FC Koln on Saturday.
Hummels will come up against his future employers in the DFB Cup final next week, but he insists he will give his all to ensure Dortmund come out victorious.
"For as long as I play against them, I want to beat Bayern. I won't spare anyone in the final," he added.