Per the club's Twitter account (in Dutch), Nouri suffered from arrhythmia. He was reported to be in stable condition and in an induced coma.
Ajax told Dutch football magazine Voetbal International that Nouri is breathing with the assistance of a machine, per Leander Schaerlaeckens of Yahoo Sports.
Andres Cordero of beIN Sports USA reported medical staff performed CPR on the 20-year-old:
AjaxDaily shared video of the incident and reported the reaction of Nouri's team-mates:
Nouri did not lose consciousness immediately, and the video showed he was breathing when medical staff made their way onto the pitch.
He received treatment on the pitch, and sheets were used to shield the view of fans and cameras.
The youngster came on in the second half of the game. He is widely regarded as one of the Netherlands' top talents and has been a regular in Oranje's youth setup.