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.
Nouri, 20, has played nine league games for the Dutch top-flight club.
"A trauma helicopter landed on the field," Ajax tweeted. "There was talk of heart rhythm disorders. He is stable."
Nouri played in the Europa League for Ajax in 2016-17 and was part of the squad which travelled to Stockholm for the final against Manchester United in May.
However, the Netherlands youth international played no part in the game, which United won 2-0.
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.
"We would like to wish speedy recovery to Abdelhak Nouri from @AFCAjax who collapsed in a friendly game," it posted on Twitter.
Nouri played a total 15 league and cup games for Ajax in 2016-17, scoring one goal in a Dutch Cup tie.
Credit:BBC Sports