Karetsas was carried off on a stretcher during Tuesday's 3-2 Champions League win over Villarreal after sitting down suddenly on the pitch without any contact from an opponent in the first half, before being taken to hospital as a precaution for further assessment.
Speaking ahead of Saturday's Bundesliga meeting with SC Paderborn, head coach Niko Kovac confirmed the 18-year-old had since undergone a full round of tests with no cause for concern identified.
"He underwent all the examinations. Thankfully, nothing unusual was found," Kovac told Sky ahead of the Paderborn match. "He already trained individually again today in Brackel.
"If things continue to go well, he could return for the Stuttgart game."
Borussia Dortmund had initially cited circulatory problems, with the player's mother suggesting dehydration may have been responsible, though managing director Lars Ricken later pushed back on that explanation, saying extensive follow-up testing over several days had found nothing abnormal at all.
The exact cause of the episode has therefore still not been officially confirmed by the club, leaving some lingering ambiguity even as the overall medical picture continues to look reassuring for all concerned.
The teenager's collapse came shortly after fellow summer signing Giannis Konstantelias suffered a serious knee injury on his own Bundesliga debut, compounding a difficult few weeks for Dortmund's new intake despite the team's strong results in both competitions so far this season.
Karetsas, who joined Dortmund for around 30 million euros from Belgian side KRC Genk over the summer, has already won 10 caps for Greece despite his age, underlining the level of talent that made his sudden withdrawal so alarming for staff and supporters alike. He remains unavailable for Saturday's match against Paderborn, with Dortmund's next fixture after that a trip to Stuttgart, for which Kovac has cautiously earmarked a possible return.
Fellow new arrival Ethan Nwaneri, on loan from Arsenal, replaced Karetsas after his withdrawal against Villarreal and has since continued in the side, including in Saturday's starting lineup, having also impressed since his own move to Signal Iduna Park.