Ajax midfielder Abdelhak Nouri is “out of danger” but remains in intensive care after collapsing during a pre-season friendly against Werder Bremen.
The incident, which happened during Saturday’s game in Hippach, Austria, led to the match being abandoned.
Ajax said Nouri, 20, had suffered “cardiac arrhythmias” – heart rhythm problems – during the game.
An open training session by the 33-time Dutch champions planned for Monday has been cancelled.
Nouri 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 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.
“We would like to wish speedy recovery to Abdelhak Nouri from @AFCAjax who collapsed in a friendly game,” it posted on Twitter.
An Ajax statement, released on Sunday, said: “Should anything change in the situation of Nouri, Ajax will report through its official channels.”