The 22-year-old midfielder has been out of action since August with a back problem.
Dortmund’s sporting director, Michael Zorc, has revealed that the midfielder’s condition is not improving.
“He is not making any progress at the moment,” Zorc told Zeitung. “We do not know when he will be able to play again.”
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Chelsea have struggled to build momentum in the opening stages of the campaign and are currently looking over their shoulders at the relegation zone.
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