Hawthorn youngster Jai Serong has the heart condition pericarditis and will not return to the AFL for two months.
The 20-year-old younger brother of Fremantle vice-captain Caleb was diagnosed on Tuesday.
It is a potentially fatal condition that causes sharp chest pain.
Serong will be restricted to walking and stretching for the next 30 days while he recovers.
"We have been working closely with a specialist sports cardiologist to ensure Jai's health is paramount to how we navigate this issue," said Hawthorn doctor Liam West in a club statement.
"The cardiologist believes Jai developed pericarditis after a viral infection that didn't show symptoms.
"Further tests will be required before Jai is cleared to begin a graded exercise program.
"Jai is no longer experiencing chest pain but is obviously disappointed that he will need a period of time out of training and match play."
The Hawks drafted Serong in 2021 and he played three AFL games last season.