Belgium's Michy Batshuayi has suffered a 'serious' injury making him a major doubt for the World Cup in June. The Chelsea loanee sustained the injury during his side's 2-0 loss to Schalke on Sunday.
The 24-year-old was taken off on a stretcher late on in the Ruhr derby, having appeared to roll his ankle.
"I didn't think it looked like the kind of injury that he would be back from quickly," Dortmund's coach Stoger said.
"I hope, of course, that I'm mistaken. I can't yet say anything specific, other than the fact that he's in pain and will now be examined in more detail. Hopefully it's not as bad as it looks."
Batshuayi has scored nine times in 14 games since joining Dortmund on loan in the winter transfer window in a deal that runs out at the end of this campaign.