The 37-year-old goalkeeper was reportedly rushed to the hospital and is currently in a stable condition.
Casillas had been working at Olival, Porto’s training facility, in preparation for their league encounter with Aves this weekend when he suffered a heart attack this morning.
According to Portuguese radio station RTP, Casillas suffered a myocardial infarction – colloquially known as a heart attack – but was treated quickly and is not in immediate danger.
He remains in hospital and is not expected to feature again for Porto in the remainder of their Primeira Liga season.
Tributes and well-wishes are being sent from around the world of football for the footballer, who won the World Cup with Spain in 2010, including three Champions League three times – with Real Madrid.
He also won five La Liga titles and two Copa del Reys with the Spanish club.