Netherlands international Daley Blind has been diagnosed with inflammation of the heart muscle, Ajax have announced.
The former Manchester United defender suffered from dizziness in Ajax's Champions League defeat against Valencia on December 10.
Although Blind completed that game, he has not played since and was last week ruled out of his club's remaining matches in 2019.
Ajax have now confirmed Blind will remain in Amsterdam to continue his recovery when the team travels to Qatar for a mid-season training camp.
"Daley Blind has been diagnosed with heart muscle inflammation," read a statement from the Dutch champions. "The central defender has undergone extensive medical examination in recent days, following the dizziness that briefly bothered him during the Ajax Valencia match.
"It was then decided to place a subcutaneous ICD on Blind, a device that is applied under the skin. As a result of this operation, which took place yesterday [Friday], the 29-year-old international will not start the training camp at the beginning of January but will continue to work on his recovery in Amsterdam."
The severity of Blind's condition has not been revealed, but the player said he was "feeling good" in a video posted on Twitter shortly after Ajax's announcement.
"First of all, I want to thank you guys for all the messages you sent me. I really appreciate it and it helped me, so [it was] very kind and thank you for that," said Blind.
"Second, Ajax just posted an update online about my condition. The most important thing is that I'm feeling good at the moment and I'm trying to come back as soon as possible.
"Again, thank you for everything and I'll see you guys soon."
Ajax are top of the Eredivisie at the halfway stage of the 2019-20 campaign, ahead of nearest challengers AZ Alkmaar on goal difference as they look to secure a second successive title.
The Dutch giants will not, however, be fighting for Champions League success next year, with last season's semi-finals having exited at the group stage after failing to finish above Valencia and Chelsea.
Erik ten Hag's side will be back in domestic action on Sunday when they welcome Den Haag to the Johan Cruyff Stadium.