Still sure that Hummels would never go to Bayern?
Or that I was wrong when I predicted that atleast 2 out of the 3 with one year left would leave?
That said. I think Hummels will make a smart decision for himself. Whether that is staying in Dortmund or transfer to Bayern or City (IMO the only 2 viable options aside from staying if Barca in fact wants Marquinhos) will be up to him. He is a pretty bright guy (atleast for a football pro) and he will weigh the Pros and Cons.
I would easily trade Dembele for Hummels, though

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You keep Hummels and give us Dembele.
Getting him on a longterm deal would be a crazy good move for Dortmund.
He is young enough that he might even extend his contract and that might give you 5+ years of IMO the hottest prospect in the world right now before he could leave for oodles of money.
Amazing move really. Hope we somehow hijack it

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If Hummels goes to Munich, I think we can assume that Goetze will absolutely head the other way.
Aubameyang/X
Reus/Pulisic/Durm - Götze/Kagawa- Dembele/Mkhi/Paßlack
Weigl/Merino - Castro/Sahin
Schmelzer/Durm - Sokratis/Subotic - Toprak/X/Ginter - Pisczek/X
Bürki/X
That is a damn good team and you would even have lots of money to spend on those few weak spots.
Whatever happens, Dortmund bshould be more than fine next season.