Dortmund have released a statement announcing that midfielder Mario Gotze will be out of action for an unspecified period due to "metabolic disorders".
A statement on the club's official website read: "Borussia Dortmund will have to do without their midfielder Mario Gotze.
"The world champion has been struggling with muscular problems in recent months, leading to a thorough internal search for possible causes.
"Investigations have now revealed metabolic disorders, which make it absolutely necessary to take Gotze out of training."
Gotze himself said: "I am getting treatment and will do everything to get back into training again to help our team reach our goals.