Liverpool midfielder Sebastian Leto's loan deal with Olympiakos is in the balance following a row with coach Ernesto Valverde.
The Argentine midfielder moved to the Greek club on a one-year loan in the summer after failing to secure a work permit to play in the Premier League.
Leto was left out of the Olympiakos squad for the weekend clash with Panathinaikos and then stepped out of line during a training session.
The 22-year-old is reported to have begun kicking footballs around wildly after refusing to train with the rest of the team, with Valverde and his assistant both struck.
Valverde told the club's official website: "During yesterday's training there was an issue with Sebastian Leto, which for the umpteenth time was a disciplinary offence - forcing me to send him to the dressing room.
"I have informed the administration about this particular incident, and it is now up to them to decide the action needed. As I have said in the past, discipline is a fundamental concern for the calmness of the team."
Leto's contract with Olympiakos included a clause that he would have to play at least 30 games this season; otherwise the Greek side would have to pay a fine to Liverpool.
The Buenos Aires-born left winger, who has impressed with his combination of pace and trickery, completed 31 games last week in the UEFA Cup match against Saint Etienne which saw Valverde's men go out 5-2 on aggregate.