Porto goalkeeper Iker Casillas has suffered a heart attack while training.
The 37-year-old, who won the World Cup with Spain in 2010, is now "well, stable and his heart problem is solved" after being taken to hospital on Wednesday.
A statement from Porto said the keeper had suffered an "acute myocardial infarction" during the morning session at Olival.
Casillas had been training in preparation for Porto's Primeira Liga game at home to Aves this weekend, although he is now unlikely to play again this season.
He burst onto the scene when he made his debut for boyhood club Real Madrid as a teenager in 1999, having come through the club's ranks. He remained at the Bernabeu for the next 16 years.
During his time at Madrid he won the Champions League three times - in 2000, 2002 and 2014 - and also won five LaLiga titles and two Copa del Rey trophies.
His former employers Real wished him well in a statement, saying they wanted to see "their eternal captain recover as soon as possible" while sending him "all the courage in the world".
Casillas moved to Porto in 2015 and helped them win the Portuguese league title last season.
Last month he played in both legs of the Champions League quarter-final defeat to Liverpool.