The Spanish goalkeeper Iker Casillas suffered a heart attack, according to Portuguese reports, while training.
According to the "Sky Sports" network in English, the Spanish goalkeeper has been transferred to the hospital for treatment and is in stable condition.
The former Real Madrid goalkeeper should miss the remaining games of the season.
The 37-year-old is in his fourth season in Portugal with Porto, who won the league title last season.
Casillas has played 42 matches for Porto in all competitions this season.
Porto are second behind Benfica, with a difference of 3 points until the end of the season.
Losing the veteran goalkeeper at the crucial moment of the season and facing the game against Avis, Nacional Madeira and Sporting Lisbon, and are also in the final of the Portuguese Cup with Sporting Lisbon.
Casillas kept a clean sheet in 18 league games of 31 games this season.
The goalkeeper of Real Madrid made all the possible titles in the real shirt to reach 20 titles, and won the titles of Euro 2008, 2012 and the 2010 World Cup with Spain.
Sky: Iker Casillas has a heart attack
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