Dembele has been skipping training as he waits for Barca and BVB to settle on a fee for his signing.
"Ousmane is a good guy, but he has to understand that no one is bigger than the team," said the Greece defender.
"Every player must work for the team and respect the hierarchy. And that applies to Ousmane as well. He cannot do what he wants. (Pierre-Emerick) Aubameyang also thought he was leaving. But he had a professional behavior at each training session and gave the best of himself.
"In the first round of the Cup, he scored three goals. And Ousmane was not there. Ousmane must not penalise himself. He must not forget that he still has a contract."