According to a report by the Belgian newspaper Het Laatste Nieuws, the Greek attacking player may have collapsed because he had not taken on enough water immediately before the match.
Doctors first suspected dehydration during an initial hospital examination. By Wednesday lunchtime, Borussia Dortmund still had not commented on any officially completed diagnostics.
Speaking to SID after being asked, a spokesman only said Karetsas was "doing well under the circumstances". Dortmund had earlier said, through sporting managing director Lars Ricken, that the 18-year-old attacking player had suffered sudden "circulatory problems".
After 23 minutes of the Champions League opener against the LaLiga club at Signal Iduna Park, Karetsas suddenly collapsed without any external contact. Moments earlier, he had gone on a completely normal dribble and played a pass. He then raised his arm and called for treatment. Shortly after that, paramedics sprinted over and carried the new signing off the pitch as the crowd applauded, with tears in his eyes.
"I went out with him, but at that moment he was not directly able to speak. We naturally hope that things get better very, very quickly and that it is nothing serious," said Dortmund sporting director Ole Book immediately after the 3-2 win. "It came very suddenly. Of course, that really puts a significant damper on the mood."
Getty ImagesWill BVB have to do without the next important player?
Losing Karetsas for any length of time would be a heavy blow for BVB. New signing Giannis Konstantelias had already torn his cruciate ligament on his first Bundesliga appearance against Hamburger SV on the opening matchday and will be out for a long time.
Before the transfer window shut, Dortmund brought in Ethan Nwaneri from Arsenal on loan at short notice as his replacement. Nwaneri came on for Karetsas on Tuesday. Dortmund are also dealing with injury concerns in defence. Kovac had to take off Nico Schlotterbeck, Emre Can and Filippo Mane in the clash against Villarreal.
Defender Ramy Bensebaini is still struggling with a bruised ribcage, but he would not have been eligible to play anyway after his second yellow red card in the match at Atalanta Bergamo (1-4) in February.