Ajax and Netherlands defender Daley Blind collapsed during Tuesday's pre-season friendly against Hertha Berlin.
Blind, 30, was diagnosed with a heart condition in December 2019 after suffering from dizziness during a Champions League game against Valencia.
The former Manchester United player returned to action in February after having an implantable cardioverter defibrillator (ICD) fitted.
Ajax boss Erik Ten Hag said Blind was feeling "OK" after being substituted.
"Daley Blind's ICD went off. He got subbed off straight away and he is feeling OK now, he is feeling fine," Ten Hag told Dutch TV station Ziggo Sport.
"We will do research about this now."