Mats Hummels says it was a 'shock' to hear Mario Gotze will join Bayern Munich

Mario Gotze: Turning his back on Borussia Dortmund

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Borussia Dortmund defender Mats Hummels admits Mario Gotze's move to rivals Bayern Munich is a "very tough blow" for the Westphalian club.

The 20-year-old is using a contract release clause to switch to the Allianz-Arena before the start of next season. In return, Dortmund will receive a transfer fee in the region of 37million Euros.

Hummels said: "It's a real blow. Mario (Gotze) is already among the ten best players in the world.

"Therefore it was a shock. I almost did not sleep the night I got to know about this. When (Shinji) Kagawa left or if (Robert) Lewandowski moves on I am able to see reasons.

"Because they are not from Germany. But this move is a very tough blow for us!"

Hummels joined Dortmund from Bayern Munich's reserve side in 2009.