Shock gave way for relief as it was confirmed that the Porto goalkeeper was in a stable condition and he was discharged from hospital less than a week after being admitted.
However, while the issue is now under control, it remains unclear whether or not the three-time Champions League winner will be able to continue playing football, with Casillas himself unable to say for sure.
"I understand that I have to be grateful, because I was very lucky," the former Real Madrid captain told reporters after leaving hospital.
"I don't know what will happen in the future, but the most important thing is to be here and let you know that I feel better."
Feeling better is one thing, but actually being fit enough to perform as an elite athlete at the highest level is another thing altogether.
Initial responses from medical professionals have been positive, suggesting that Casillas could potentially return to action, but it will be a number of months before doctors can give the player a definitive answer.
"It’s going to depend on many factors: what kind of medication he needs, how he holds up not just at rest but also under stress and during the physical exercise required by his job, and on his own determination to carry on,” said Porto doctor Pulga.
"It’s only after pondering these factors calmly and through a lot of dialogue that a decision will be made whether he wants to continue."
Juan Antonio Corbalan, a cardiologist and former Real Madrid basketball player, is not as optimistic as Pulga though, telling RTVE: "You cannot play football with a stent. And that particularly applies to a goalkeeper.
"Iker will end up returning to enjoy a normal life but I very much doubt he will play professional football again."
With a number of footballers suffering fatal heart-related illnesses while playing - the most high profile examples of recent years being former Manchester City midfielder Marc-Vivien Foe and ex-Newcastle United player Cheick Tiote - the serious concern for Casillas' wellbeing is totally understandable.
Former Barcelona goalkeeper and Casillas' compatriot Victor Valdes has even urged his old team-mate to hang up the gloves for good.
In a message shared on his official Instagram page, Valdes wrote: "Your heart, that continues beating for the sake of those who dream of reaching what you once were, as a friend and as a rival in times gone by, I encourage you to be that white lion once again but this time on the sideline."
Despite the ominous forecasts, however, Casillas' agent, Carlos Cutropia, says that retirement "is not something we're worrying about at all."
There is cause for optimism, not least because some footballers have even recovered from major surgeries to return to the pitch. Cristiano Ronaldo had minor heart surgery as a teenager before going on to break record after record with Manchester United, Portugal and Real Madrid.
There have been other, less heralded, examples too. Republic of Ireland international Shane Duffy, who now plays in the Premier League for Brighton, underwent life-saving surgery on his liver in 2010 after colliding with a goalkeeper, while former Derby County defender Mark O'Brien played in the Championship after having a major heart surgery.
However, each of the aforementioned examples were in their teens when they experienced that trauma and were arguably more resilient, naturally, than a 37-year-old Casillas might be.
Either way, the World Cup winner will have wait to learn the full extent of his heart problem and then he will have to make a decision; the biggest one of his career - and his life.