The legendary Spanish goalkeeper, 37, is said to be recovering well in hospital after suffering the attack in training on Wednesday.
Porto, second, host Desportivo Aves on Saturday in their third last league game of the season, trailing leaders Benfica by two points.
"When it happened I must say we didn't think it was very serious, we thought it was just some indisposition like (the kind that) sometimes happens," Porto coach Sergio Conceicao told reporters on Friday.
"In the end we had the news, after 30 minutes the doctor called me and that was when I felt the team was shocked with what (had) happened.
"In that moment we were apprehensive, we had knowledge of the seriousness of the situation, but fortunately Iker is well.
"The team is with him and extremely united with him and his family because football is part of life, but life itself is much more important than anything else."
Casillas played 725 times for Real Madrid, winning thee Champions Leagues and five La Liga titles, as well as the World Cup with Spain in 2010 and European Championships in 2008 and 2012, joining Porto in 2015.
Reserve goalkeeper Vana is poised to take Casillas's place in the side.
Benfica host Portimonense earlier on Saturday, needing to win all three of their remaining matches to guarantee their 37th Primeira Liga title.
(Reporting by Rik Sharma; Editing by Christian Radnedge)
