Dembele has agreed a move to Barca in an initial €105million switch after refusing to train for Dortmund, who then suspended the France international prior to his move.
"Of course we talked about it, but I wish him all the best," he said. "He knows for sure that he made a mistake [with the nature of his departure] and that it might have worked differently.
"I can understand him, that he wanted to change to Barcelona. I went to [Real] Madrid myself. He is a fantastic footballer and I wish him all the best. I'll be watching when he plays.
"We are not Barcelona. We have talked about the following: if Barcelona lose Neymar, we cannot hide from it at Dortmund.
"In world football, it is so that there is always a step above your own club. We also buy players from other clubs. The roundabout will continue to turn.
"But Borussia Dortmund will always continue. We are playing good football right now."