Gonzalo Castro has hit out at Ousmane Dembele's behaviour as the young winger seemingly attempts to force through a move to Barcelona.
The Spanish club are aiming to replace Neymar this summer after the Brazilian joined PSG for a €222 million fee earlier in the transfer market.
Barca have identified Dembele as a potential acquisition, but Dortmund, who have suspended the player after he missed training, have thus far resisted the Catalan giants' overtures.
And the 20-year-old has now had his actions questioned by his more senior team-mate, the 30-year-old Castro, who admits that he was taken aback by the forward's behaviour.
"We were shocked [at Dembele's decision to miss training]. That was news for all of us. This is something I have never experienced," he told Bild. "We cannot forget that he is only 20 years old and nobody knows who is behind his actions. At his age, I could not decide everything by myself.
"We have to wait and see if he's going to show up again. There must be a clear sign by him [that he wants to return to the team].“
Dortmund face Wolfsburg in the Bundesliga on August 19 as the new domestic season kicks off in Germany.