The tests revealed he has heart muscle inflammation, also known as cardiomyopathy.
The 29-year-old, who spent four years at Old Trafford, has since been fitted with a subcutaneous implantable cardioverter defibrillator.
He has missed Ajax’s last two matches of the years and is not expected to rejoin training in January as he continues with his recovery.
An Ajax statement posted online said: “Daley Blind has been diagnosed with heart muscle inflammation.
“The central defender has undergone an extensive medical examination in recent days, following the dizziness that briefly bothered him during the Ajax v Valencia match.
“It was then decided to place a subcutaneous ICD with Blind, a device that is applied under the skin.”
In the wake of his diagnosis, Blind said: “I feel good at the moment.
“I am trying to come back as soon as possible.”
The severity of the condition currently remains unknown.