Former New Zealand centre Sonny Bill Williams said in a video published on Friday that his heart "was about to pop" before undergoing life-saving surgery.
Williams had discovered earlier in the year that he had an undetected heart defect from birth as he underwent a neck operation.
The 41-year-old won the Rugby World Cup in 2011 and 2015 as well as Australia's National Rugby League in the 13-man code in 2004 before retiring five years ago.
On top of a trophy-laden rugby career, Williams was also a professional boxer, winning nine out of 11 bouts.
"The doctors both said it was a miracle," Williams said on YouTube.
"My heart was about to pop, the doctors said.
"If I hadn't gone in and got it checked, they wouldn't know, it would have been a case of a perfectly healthy person dropping dead."
One of Williams's surgeons said the Ross procedure to replace a valve took more than four hours.
"You'd been playing with a handicap which you hadn't realised at the time," Michael O'Keefe said.
"The fact you did so just goes to show how fit you were to be able to compensate for it," he added.
Last week, former South Africa under-20s lock Hyron Andrews was found dead aged 31 in the hotel of his team, the Lions, after a tear in his aorta artery which carries blood to and from the heart.