Brazilian midfielder Arthur Melo could face sanctions for failing to turn up at training with Barcelona, who concluded a deal to sell him to Juventus a month ago.
The Catalan club say the player is under contract until the end of the season and have demanded he return, with the local press claiming he was in his native Brazil.
Arthur has been sold to Juve in a deal worth 72m euros ($81m) with Bosnian playmaker Miralem Pjanic moving in the opposite direction for a similar fee at season's end.
The player had been reluctant to leave the Camp Nou and even released a statement in April, insisting his "idea was clear" and that he wanted to play at Barca "for many years".
Spanish sports daily Mundo Deportivo say the player maintains the then coach Quique Setien had told him he had played his last game with the La Liga club.
Arthur has not featured for Barca since the transfer was announced on June 29.
Barca resumed training Tuesday ahead of their Champions League last-16 second leg with Napoli next month.