Porto later confirmed that the goalkeeper was out of danger and is expected to make a full recovery, although he will play no further part in the club's Primeira Liga campaign.
Doctors, however, fear that Casillas' heart condition may have put an end to his illustrious playing career that has spanned almost two decades with Madrid and the Portuguese giants.
“We will have to wait to see how his body heals and then study the results of the rigorous medical tests he will be put through,” Dr Jose Gonzalez told AS. “His heart could return to functioning as normal and he might have the chance to return to his profession.”
Cardiologist Jose Calabuig struck a similar tone between caution and optimism, telling Cadena SER's El Larguero: “Physically, if there is no scarring then there's a chance the muscle will heal. Within a timeframe of three to six
“If all goes well then he will certainly be able to return to work.”
Casillas remains a favourite at the Santiago Bernabeu having made more than 700 appearances for the Merengue between 1999 and 2015, and prior to the club's clash against Villarreal it was made it clear that the veteran was in the players' thoughts.
"Iker, we are with you," read the legend on the Madrid first XI's T-shirts as they lined up for Sunday's game, which eventually finished 3-2 in the hosts' favour thanks to a brace from Mariano.
Madrid's official magazine Grada Blanca echoed that message, with the front cover sporting a photograph of Casillas from his glory days at the club.
Casillas, who turns 38 on May 20, made his debut in the Madrid first team aged just 18 and has gone on to appear more than 1000 times at the top level for the Blancos, Porto and the Spain national team.
His trophy-laden career has seen him lift six national league titles in Spain and Portugal, three Champions Leagues, two European Championships and the 2010 World Cup among a total of 26 major titles.