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.