Ajax's preseason friendly against Werder Bremen was abandoned after Abdelhak Nouri required emergency ambulance treatment on the field.
Nouri, 20, collapsed with around 20 minutes remaining in the game in Austria and there was immediate concern for his condition.
An ambulance and even a helicopter were called onto the pitch while the ground was cleared, though Nouri was eventually airlifted to hospital in what Ajax described as a "stable'' condition.
Nouri wordt nu per helikopter naar het ziekenhuis gevlogen. Ontzettend bedankt voor al jullie steunbetuigingen.
- AFC Ajax (@AFCAjax) July 8, 2017
A series of tweets from Ajax's official account read: "Alarm on and around the field. There is something going on with Abdelhak Nouri. But it's not clear what.
"An ambulance is coming onto the field. It looks serious, we hope soon for more info on Abdelhak Nouri.
"Emergency services are still with Nouri, players look worried. It is serious, we will let you know immediately if there is more clarity.
"Meanwhile a trauma helicopter landed on the field. People are asked to leave the stadium.''
Nouri reportedly received CPR on the field before leaving in the helicopter.
Around 15 minutes after the news first broke, Ajax tweeted: "Update | Nouri had heart arrhythmias. He is stable, has a heartbeat and is asleep. He is going in the helicopter to hospital. Thanks so much for all your support.''
Nouri made 15 appearances for Ajax's first team last season, including three in the Europa League, and scored on his senior debut in a 5-0 win over Willem II in the KNVB-Beker.
Bremen led 2-1 when the friendly in Zillertal was abandoned, though the scoreline was instantly overshadowed by Nouri's condition.