Karetsas had just completed a counterattack when he called for attention and sat down on the field before being laid down and receiving medical attention.
The home crowd fell silent, but gave Karetsas a standing ovation as he was carried off. By then, 23 minutes of the Champions League match had been played.
Borussia Dortmund wrote on X that Karetsas suffered from circulatory problems. "Under the circumstances, Kosta is doing well and is on his way to hospital for further examinations."
Konstantinos Karetsas came to Dortmund this summer from Belgian club Genk for 30 million euros, equivalent to approximately 335 million kronor.
According to Bild newspaper, the 18-year-old skipped Dortmund's training session on Monday due to fatigue.
