Barrios could miss six weeks
Dortmund club doctors had previously indicated that the 26-year-old would be out for between two and three weeks after suffering a thigh muscle tear during Paraguay's Copa America final defeat to Uruguay on 24 July.
"The scans that we were sent meant we were already preparing for the news that the injury would be more serious than first feared," admitted Dortmund coach Jurgen Klopp today.
Barrios scored 16 goals during Dortmund's triumphant run to the Bundesliga title last season. His team begin the defence of their crown without him against Hamburg on Friday evening.