The Germany international went onto say that Gundogan wanted the move for ‘some time’ and that he wishes him all the best at his new club.
“We all knew for some time that this is what he wanted,” Mats Hummels was quoted as saying by Goal.
“I am happy for him that it worked out now, because he decided that he wants to do it and his injury kind of questioned this again.
“So I hope for him that he made a good decision and especially that he will be fit again soon.
“Because if you’re not fit it doesn’t matter where you are: you won’t play anyway.”
Dortmund are understood to be lining up a replacement for Gundogan who joined City earlier this week.
According to Transfermarkt the Colombian midfielder Jefferson Lerma is being courted by Dortmund and that the German club will make a move for Lerma if Gundogan departs.
Gundogan has been ruled out of Euro 2016 and the beginning of next season after dislocating his knee during training earlier this month.
The German tabloid Bild report that it will take Gundogan four to six months for him to make a full recovery.