Borussia Dortmund have travelled to Azerbaijan for the Champions League match against Sabah on Tuesday without midfielder Konstantinos Karetsas.
"Kos has a ligament injury in his foot. He will not travel to Baku and will also be unable to play against Eintracht Frankfurt (on Saturday)," managing director for sports Lars Ricken told reporters ahead of departure on Monday. "We have to take it day by day."
The 18-year-old suffered the injury during the goalless draw in the Bundesliga with Union Berlin at the weekend.
"That's obviously annoying. But to be honest, we were also fearing worse, seeing how his team-mates reacted when he was on the ground and limping off the pitch," Ricken said.
The game in Baku will conclude an intense week of travelling for Dortmund. With last Wednesday's match against Bodø/Glimt in Norway, and the trips to Berlin and Baku, Dortmund have travelled around 11,300 kilometres.