Michy Batshuayi is in "great pain" with an injury sustained in the derby defeat to Schalke, Borussia Dortmund coach Peter Stoger has confirmed.
Batshuayi was carried off on a stretcher towards the end of Dortmund's 2-0 loss to Ruhr rivals Schalke in the Bundesliga on Sunday.
The Belgium striker - who has hit seven league goals since joining on loan from Chelsea in January - covered his face with his hands as he left the field.
And Stoger, while unwilling to offer a diagnosis for Batshuayi's apparent ankle injury, accepted it could be a serious blow for the forward, who was seen leaving the stadium on crutches and was reportedly taken to hospital.
"He is in great pain," Stoger said. "It looked like he could be out for a long time. I hope it's not that bad."
Second-half strikes from Yevhen Konoplyanka and Naldo earned bragging rights for Schalke, who moved four points clear in second place thanks to their first win over BVB since 2014.
Stoger left Mario Gotze on the bench until after Naldo's 82nd-minute thunderbolt secured all three points, preferring teenager Jadon Sancho to the Germany star in his second substitution.
"Mario is a talented player," Stoger added. "But I have to give another chance to the team from the Stuttgart game after the good performance [a 3-0 win].
"We also have many other young players who can come from the squad. That's just as true for Mario as it is for them.
"We did not have the attacking impact, which you need in such a derby. You do not have to be surprised if you leave the field as a deserved loser."