Borussia Dortmund coach Niko Kovac was left frustrated by the referee's decision not to send Kelsey Owusu off for a robust challenge on Yan Couto.

Dortmund ran out 1-0 winners over third-tier Essen in the DFB-Pokal on Monday.

After facing stubborn resistance, Dortmund finally found the breakthrough 11 minutes from time through striker Serhou Guirassay.

But it was a challenge late in the game from Owusu on Couto, which led to the latter being stretchered off the field by paramedics, which dominated the post-match reaction, with Kovac puzzled that referee Frank Willenborg deemed it worthy of only a yellow card.

"I can’t let it stand like this," said a furious Kovac.

"I have to be honest, that's not how you go into a duel. I don't want to accuse him of any intention, but that's grossly negligent."

"Of course, we don't have a choice, the referee didn't see it that way. But that's not how you go there.”

Kovac was unable to give a clear prognosis of the injury, but hoped Couto was just suffering from a heavy bruise instead of a more serious problem.

He said: “I hope that not so much has happened.

“He is in severe pain. We have to wait for the investigation. I'm confident that maybe it's just a severe bruise.

"We'll examine what happened. I hope nothing's broken."

Julian Brandt also criticised the challenge and spoke of his concerns over Couto's injury, given Dortmund's small squad size.

"This is brutal," the Germany international said.

"First of all, we're a bit lucky that he doesn't stay completely on his leg, but that it's more of a grazing shot. I think otherwise it looks really bad for him.

"I haven't seen it yet. I hope he got off lightly. I hope that we don't lose Couto now, because we're not the big squad at the moment."

Kovac and Dortmund must now prepare for their first Bundesliga match of the season, away to FC St. Pauli on Saturday.