After spending eight years at Dortmund, Hummels could expect a frosty reception when he returns for the Super Cup. But the 27-year-old clarified his decision, claiming he has joined one of the world's best teams.
Bundesliga champions Bayern Munich unveiled their two summer signings as preparations are ramped up a level before the season kickoff on August 26.
Renato Sanches and Mats Hummels have strengthened Carlo Ancelotti's options this summer as Bayern aim for their fifth consecutive championship. Sanches joined the club for around 35 million euros ($38.8 million), while Bayern invoked the clause on Hummels' contract at Borussia Dortmund. CEO Karl-Heinz Rummenigge confirmed the fee wasn't 38 million euros, as originally reported.
The German international center-back teams up with Jerome Boateng this season, his defensive partner at the European Championship. Hummels is returning to the club where his career started, and Bayern's international standing played a role in his decision to leave Dortmund.
"FC Bayern are one of the three best teams in the world. Knowing that every year you can win three titles is a great attraction," Hummels said. "The decision to move took a lot of strength and it gave me a headache for several weeks - it was a long and complex process."
Joshua Kimmich, Kingsley Coman, Robert Lewandowski, Thomas Müller and Manuel Neuer are still to return to training, but Ancelotti expects all to feature in the German Super Cup clash with Dortmund next Sunday.
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Meanwhile, Borussia Dortmund midfielder Julian Weigl said he didn't need to offer help to the club's youthful crop of summer additions. The 20-year-old, who joined last season from 1860 Munich, is familiar with the experience of moving to a new club, but wants to take his game to a new level this season.
Raphael Guerreiro (22), Ousmane Dembele (19), Emre Mor (18), Mikel Merino (20) and Marc Bartra (25) have been brought in, while 24-year-old Mario Götze returns to the club alongside 25-year-olds Andre Schürrle and Sebastian Rode.
"I played a lot of games last season in a central position, where you have to be very vocal," said the midfielder at a press conference. "I think that I can be an important link between the young players and the others. I have been here for a year now and need to show that I am up to my performances of last year.
"I want to develop even more, to try and place that deadly pass and not just go for the easy option."
Despite losing Hummels, Ilkay Gundogan and Henrikh Mkhitaryan, Bayern chief executive Rummenigge still sees Dortmund as Bayern's main challenger. The yellow-and-blacks meet the German champions on Sunday before an away trip to Eintracht Trier in the German Cup. Dortmund's first league encounter is at home to Mainz on August 27.