Yes, based on news reported from several sources, Hummels will indeed leave Dortmund as a free agent. This news has been confirmed and is on the @FabrizioRomano account.
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This is confirmed news and not that big of a surprise I think. What I didn't know is how fierce the argument was between Hummels and Terzic, but the media got it wrong when they first said that Hummels won the fight, and then hours later they had to realize that not only Terzic is going to leave the club, but Hummels too. So it made no sense after all. Nobody won any fight actually.
Yes, it is related to the violent confrontation between Hummels and Terzic in the week of the UCL final. Maybe because Hummels is a senior player there and really understands various things, then he criticized Terzic regarding the tactics he had prepared. hemm, this was quite exciting at that time. In fact, it is a very decisive event.
But never mind, the point is that no one wins or loses. yes, both of them left Dortmund. And now, Nuri Şahin as the new coach of Drotmund has been officially confirmed and they are getting started with several players to get in the transfer market later.