Ajax midfielder Abdelhak Nouri collapsed and was taken to hospital by helicopter during a pre-season friendly against Werder Bremen.
The incident, which happened in the second half of the game in Austria, led to the match being abandoned.
Ajax later said the player was “stable” in hospital and being “kept asleep for optimal treatment”.
Nouri, 20, was part of the Ajax squad for the Europa League final against Manchester United in May.
However, the Netherlands youth international played no part in the game, which United won 2-0.
Nouri played a total 15 league and cup games for Ajax in 2016-17, scoring one goal in a Dutch Cup tie.
Former Netherlands captain Ruud Gullit posted on social media: “#abdelhaknouri we pray for you.”
Ajax Cape Town, a feeder club for the Dutch side, also sent a message of support.