Porto goalkeeper Iker Casillas has been hospitalised after suffering a heart attack during training.
The Spaniard has reportedly already undergone surgery, and his life is not believed to be in danger.
A Porto statement said: "Iker Casillas suffered an acute myocardial infarction during morning practice this Wednesday, at the PortoGaia Sports Formation and Training Centre, in Olival.
"The work session was promptly interrupted so that assistance could be given to the FC Porto goalkeeper, who is at this moment at the CUP Porto Hospital. Casillas is fine, stable and with his cardiac problem resolved."
The 37-year-old is regarded as one of the greatest goalkeepers of all time, having captained Spain to the World Cup in 2010, and two European Championship titles, in 2008 and 2012.
He made his professional debut for Real Madrid as a teenager in 1999, and went on to make more than 500 appearances for the Spanish giants, winning three Champions Leagues and five La Liga crowns.
He left Madrid in 2015 to join Porto, with whom he won the league title last season.
Casillas signed a new one-year deal with the Portguese club in March, revealing his desire to remain with the club until he retires.
The World Cup winner will miss the remaining three games of Porto's title run in, as they look to bridge a two-point gap to leaders Benfica, as well as the Portuguese cup final against Sporting.
