The 18-year-old Greece international suddenly went down during the first half at Signal Iduna Park after passing the ball to a teammate. He immediately signalled to the referee that something was wrong before sitting on the pitch and receiving treatment from Dortmund’s medical team.
Karetsas was eventually carried from the pitch on a stretcher, with the incident causing visible concern among his teammates and supporters. Dortmund later confirmed that the youngster had experienced circulatory problems and was taken to hospital for further tests.
Dortmund provide update on Karetsas
Dortmund said Karetsas was “doing well under the circumstances” while confirming that he was on his way to hospital for additional examinations. No definitive diagnosis had been announced immediately after the match.
Dortmund sporting director Lars Ricken said the problem appeared to have occurred suddenly, with no obvious warning signs beforehand. Club officials indicated that they would wait for the results of further medical examinations before providing more information about the teenager's condition.
Dortmund goalkeeper Gregor Kobel also expressed his hope that Karetsas would make a quick recovery.
• Gregor Kobel: “I hope and I think that everything is OK and I hope that he’s back and fit with us soon.”
Karetsas making Champions League debut
The incident came during Karetsas' first appearance in the UEFA Champions League, having joined Dortmund from Belgian side Genk during the summer.
The Greece international was regarded as one of Dortmund's major young signings and had arrived at the Bundesliga club for a reported €30 million fee.
Karetsas' medical scare overshadowed what eventually became a memorable night for Dortmund, who recovered from a goalless first half to defeat Villarreal 3-2.
• Borussia Dortmund: Renato Veiga 53' (own goal), Serhou Guirassy 80', 85' (pen)
• Villarreal: Santiago Mouriño 66', Serhou Guirassy 90+2' (own goal)
Despite the late scare, Dortmund held on for three points in their opening Champions League league-phase fixture.
The immediate priority, however, remains Karetsas' health, with the club awaiting the results of the further examinations carried out after his hospitalisation.