Daley Blind collapsed and was taken from the field during Ajax's pre-season friendly against Hertha Berlin on Tuesday.
The 30-year-old dropped to the ground during the latter stages of the match with concerned teammates rushing to his side.
Blind was diagnosed with inflammation of the heart muscle in December 2019 after suffering dizziness during Ajax's Champions League defeat against Valencia.
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The ex-Manchester United star was subsequently fitted with an implantable cardioverter defibrillator (ICD) after the incident and returned to full training in February.