Michael Olaitan was taken to the hospital after collapsing during the first-half of Olympiacos’ 3-0 loss to Panathinaikos on Sunday.
The striker was running off the ball unchallenged when he fell to the pitch.
He regained consciousness before being taken to the hospital and was later reported to be in a very stable condition.
First examination on the Nigeria youth international revealed he had collapsed due to a viral myocarditis caused by an infection.
The Athens derby was goalless when the 21-year-old fell before Panathinaikos’ Croatian midfielder Danijel Pranjic scored.
Marcus Berg doubled the visitors’ lead in the 67th minute before Mehdi Abeid completed scoring for Panathinaikos, scoring with a free-kick, seconds to the dead.
despite their first league loss of the season, the runaway Greek Super League leaders, Olympiacos, who visits Old Trafford for the return leg of their Champions League last-16 tie against Manchester United on March 19, remain 19 points clear at the summit of the log.
Panathinaikos are five points adrift of their city rivals in fourth place.
Olaitan was pivotal to the Greek champions’ 2-0 win over United last Tuesday.
Another Nigeria youth international, Abdul Ajagun, was on for 87 minutes in Panathinaikos’ win.
Panathinaikos manager Yannis Anastasiou earlier fell to the ground after being hit with an object thrown from the crowd but was stable to celebrate his team’s goals.