Karetsas was stretchered off the pitch in the 27th minute of Tuesday's 3-2 Champions League win over Villarreal, having sat down without any contact from an opponent moments after completing a routine dribble and pass.
The 18-year-old Greece international, making just his fifth appearance since his summer move from Genk, was accompanied off the field by emergency doctors and taken to hospital.
His mother, Irena, has since confirmed to Belgian outlet Nieuwsblad that the initial fears have eased.
"Kos was allowed to leave the hospital, which is a huge relief. The club is being very cautious and wants to rule out every possibility through various tests," she said.
Irena added that the circulatory problems appear to have stemmed from dehydration rather than anything more serious. Karetsas, who joined Dortmund for around €30 million on a five-year deal after a standout campaign at Genk in which he set up 11 goals in 28 appearances, was replaced by fellow new arrival Ethan Nwaneri, who himself only joined the club on deadline day.
Elsewhere at the club, Hans-Joachim Watzke has revealed he seriously considered stepping down as Borussia Dortmund president around the time of his election to the role in November 2025.
"The way things turned out, with one intrigue or another being run against me too, it was a difficult year for me, the hardest of the 20 years," Watzke said, according to Bild, in an interview for the Sky documentary series Meine Geschichte, broadcast on Friday evening.
Watzke was elected with just 59% of the vote, a notably weaker mandate than he had anticipated after two decades leading the club as managing director.
"And that this happens at BVB after 20 years, that really affected me. That's why it was so hard. [...] I was never a dictator," Watzke said, revealing he only decided against withdrawing his candidacy on the way to the podium to deliver his acceptance speech.
"The level of outrage gave way, step by step, to the level of responsibility," he explained of that moment.
Watzke, 67, spent almost 21 years as Dortmund's managing director before stepping down from that post last autumn to take on the presidency, having steered the club through its brush with insolvency in 2005 and two Champions League finals along the way.