Borussia Dortmund midfielder Mario Gotze has been ruled out of action for the foreseeable future after the cause of his long-term muscle problems was found to be linked to his metabolism.
The 24-year-old Germany international, whose playing time has been restricted to 24 minutes since the start of the year, returned to Dortmund in the summer after spending three years with Bayern Munich.
The Bundesliga club said medical tests revealed Gotze's muscle problems had been traced to what they described as "metabolic disturbances".
A statement on Dortmund's website read: "Bundesliga club Borussia Dortmund will continue to be without midfielder Mario Gotze for the time being.
"The 24-year-old World Cup-winner has continually suffered with muscular problems in the last few months, prompting a comprehensive internal investigation into the possible causes.
"Those investigations revealed the player has metabolic disturbances, rendering it absolutely necessary to withdraw him from team training for the time being."
Gotze said: "I'm currently undergoing treatment and will do everything in my power to be back in training and helping my team to achieve our common goals as soon as possible."
Dortmund's sporting director Michael Zorc added: "We're happy to have now uncovered the reasons for Mario's difficulties and are convinced that his extraordinary abilities will give us additional quality once he has completely recovered from this. Mario has the full backing and maximum support of everyone at BVB during his recovery."