Karetsas, 18, felt a circulatory problem during the 3-2 win over Villarreal and was taken to a hospital in Dortmund. The German club is still awaiting the results of his examinations for a better evaluation of the case. This is not the first time Borussia Dortmund has seen a new signing struggle with injuries, with midfielder Giannis Konstantelias already ruled out for the season due to a ruptured anterior cruciate ligament (LCA) in the Bundesliga.
Karetsas, who joined the club in June for 30 million euros, had been present in all of Borussia Dortmund's matches at the start of the season. However, he only lasted 27 minutes in the Champions League match before being substituted. The club has stated that Karetsas is "well, considering the circumstances," but is still awaiting further examination results.