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.
Ajax coach Erik ten Haag confirmed that there was an alert sent by Blind's ICD as he fell to the pitch during the Hertha match.
"Daley Blind's ICD went off," Ten Haag said to Ziggo Sport. "He got subbed off straight away and he is feeling OK now, he is feeling fine. We will do research about this now."
Having already spent two months out of the game following the initial heart diagnosis in December, Blind faces another spell on the sidelines while doctors ascertain what caused the collapse.
Born in Amsterdam, Blind joined Ajax's youth academy at the age of eight and worked his way up the ranks before making his debut in 2008. He spent six years at the club in a variety of midfield and defensive roles before earning a move to Manchester United in 2014.
Blind returned to Ajax in 2018 and was a part of their famous run to the Champions League semi-finals in 2019, where they fell short against Tottenham courtesy of Lucas Moura's dramatic winning goal.
The initial heart incident last year came as a shock with Blind's senior career featuring more than 350 matches across 12 years including 69 caps for the Netherlands. Across his career, he has won six Eredivisie titles with Ajax while also lifting the KNVB Cup on one occasion.
Blind failed to win the league while at United, but was a part of four trophy triumphs including the Europa League, FA Cup, Carabao Cup and Community Shield.
Ajax are currently preparing for the start of the 2020-21 Eredivisie season, after the previous campaign was abandoned with no winner due to coronavirus.