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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Since Guus Hiddink arrived at the helm, Chelsea have improved considerably after a tricky start to the campaign.
Newcastle United are staring down the barrel of relegation to the Championship for the second time in the past eight years.