Olympiacos striker Michael Olaitan collapsed during his side's Greek Super League match against Panathinaikos at the Karaiskakis Stadium on Sunday.
The 21-year-old fell to the ground when he was running off the ball in the first half and was taken to hospital after regaining consciousness.
Olaitan is now in a stable condition and has not suffered any long-term damage.
Olympiacos, who defeated Manchester United 2-0 at home in the first leg of their Champions League last-16 tie last month, lost the Athens derby 3-0.
