Defending Bundesliga champions Borussia Dortmund have learnt Lucas Barrios could be out for up to six weeks after it was confirmed his thigh injury is more serious than first thought.
The Paraguay striker sustained the muscle tear during his country's Copa America defeat to Uruguay at the end of July.
Dortmund club doctors had previously indicated the player, 26, would be out for between two and three weeks.
But manager Jurgen Klopp said: "The scans that we were sent meant we were already preparing for the news that the injury would be more serious than first feared."
Barrios scored 16 goals during Dortmund's triumphant run to the Bundesliga title last season.
His team begin their defence of their crown without him against Hamburg on Friday evening.
Meanwhile, Dortmund general manager Michael Zorc has confirmed Ecuador youth international full-back Cristian Ramirez has arrived in Germany for a trial.