The 20-year-old has been heavily linked with a move away from the club this summer following interest from Barcelona and Real Madrid and it appears that the speculation over his future may be going to his head.
The France international did not attend training today, with manager Peter Bosz unable to explain the player's absence.
"Ousmane Dembele was not in training today. We do not know why. We tried to reach him but we did not," he explained.
"First I have to talk to Dembele, then he has to explain to me why he wasn't here today. I hope nothing serious happened."
Dembele shone in the German top flight last season and Dortmund are reluctant to let the player go but if he continues to act in this manner the club may be left with no choice.
Bosz: "Ousmane Dembele was not in training today. We do not know why. We tried to reach him but we did not." #fcrbvb
— Stefan Buczko (@StefanBuczko) 10 August 2017