The 18-year-old had only recently entered the match when he went down in pain while trying to drive towards the byline. His foot caught in the turf during an exchange involving Union captain Rani Khedira, leaving him unable to continue.
Borussia Dortmund’s official account said Karetsas had been injured five minutes after coming on as a substitute. The club’s managing director for sport, Lars Ricken, confirmed that further examinations would be needed before the severity of the problem could be established.
“He twisted his ankle. Something’s not quite right with it. We’ll have to do further testing to determine how serious the injury is,” Ricken said.
Sporting director Ole Book also expressed concern, adding: “Let’s hope it’s not too serious. At the moment, I don’t feel good about it.”
The injury was a frustrating interruption for Karetsas, who had brought fresh energy to Dortmund’s attack after entering the game in the second half. He completed four of his six passes and won four of five duels before being forced off.
His latest setback also comes after a worrying incident during Dortmund’s Champions League campaign. Karetsas collapsed during the opening match against Villarreal, prompting renewed attention to his physical condition.
Head coach Niko Kovač acknowledged that the earlier episode remained on the teenager’s mind.
“We all still remember the situation when Kostas collapsed during the Champions League match. I think it’s still in the back of his mind. You can’t ignore all of that,” Kovač said.
“I’m keeping an eye on him, and he’s not yet where he was before.”
Dortmund signed Karetsas from Belgian club Genk during the summer, and he has made five appearances for the German side. His development has been watched closely as he adapts to a new league and the demands of senior football.
The timing of the injury is particularly unfortunate given the contribution he made after coming off the bench against Union. Dortmund were unable to find a breakthrough in Berlin, extending a run of matches without a win.
The club have not yet confirmed a diagnosis or a recovery timetable. Until the further tests are completed, it remains unclear how long Karetsas could be unavailable.