He then returned to training six months on, after making a recovery, but didn't play for Porto again.
The legendary Spanish 'keeper announced his retirement from football earlier this year in July.
Speaking about the traumatic episode in his life, Casillas revealed that former manager Jose Mourinho was one of the first people to contact him after hearing about the heart attack.
The 39-year-old also revealed that Mourinho enquired about his wife Sara Carbonero, who was diagnosed with ovarian cancer just three weeks on from his heart attack.
"You can become closer to people perhaps you weren't as close to before," he told ESPN Desportes.
"A lot of people don't know for example that my old manager Mourinho was the first person who was concerned with what happened to me and then later with my wife."
Iker Casillas and Jose Mourinho shared a rocky relationship at Real Madrid
Casillas, who was captain of Real Madrid when Jose Mourinho took over, shared a rather tempestuous relationship with the Portuguese manager.
He was, later on in Mourinho's tenure at the club, even dropped in favour of Diego Lopez.
However, Casillas revealed that he had put their differences behind him and lived each day on its merit, completely in the present.
"I've seen people again that I hadn't seen for a long time. You value the day-to-day more. You're not stuck thinking about what to do tomorrow or next week. It's just about what today will bring and enjoy the moments more. I was very lucky for whatever reason. Others have not been.
"I felt that I was definitely going to die that day, and the circumstances forced me to reconsider everything. I'm a privileged person."
Casillas hung up his gloves after a trophy-laden career, winning the World Cup and two European Championships with Spain, in addition to three Champions League and five La Liga titles with Real Madrid among other silverware.Published 11 Oct 2020, 12:41 IST