Sebastian Kehl will be out for up to four weeks with a shoulder injury sustained during Borussia Dortmund's 1-0 friendly victory over Steaua Bucharest on Saturday.
The defensive midfielder left the field in the 35th minute of the match in Alicante, before Jakub Blaszczykowski scored the winner for the Bundesliga strugglers six minutes from time.
Kehl's layoff is as a blow to coach Jurgen Klopp as he bids to haul his side away from danger in the second half of the season.
Dortmund are currently second from bottom, ahead of Freiburg on goals scored, and have just two wins from their last 14 Bundesliga matches.
Klopp's men face two more friendlies, against Utrecht and Fortuna Dusseldorf, before returning to Bundesliga action at Bayer Leverkusen on January 31.
