Porto are on 79 points with two games left in the campaign, while leaders Benfica have 81.
Porto's fans paid tribute to Casillas before their game against Aves by chanting the Spaniard's name and unfurling a banner with an image of the goalkeeper above the message: "Stay strong, Iker".
Casillas, who captained Spain to their only World Cup triumph in 2010 and played 725 games for Real Madrid before joining Porto in 2015, has been recovering in hospital since having a heart attack in a training session on Wednesday.
Mexico international Jesus Corona gave Porto the lead in the 18th minute while Francisco Soares doubled their advantage on the half hour mark from the penalty spot.
Soares struck again in the second half to score Porto's fourth goal, moments after Wilson Manafa had made it 3-0.
"It has been a difficult week but the players reacted positively," said Porto coach Sergio Conceicao.
"This victory goes out to Iker and we send him a very big hug, we are all in this together. In these types of situations it's not about clubs or anything, it's about people.
"Today was about a person who had a big problem and this was a great show of support for Iker. We have had many years with him and we feel a lot for him as he's a great player and an excellent human."
Benfica went behind in their game when Bruno Tabata scored for the visitors in the 56th minute.
But Rafa Silva responded by scoring twice in the space of four minutes to give them the lead while Swiss striker Haris Seferovic also netted a quick-fire double before Brazilian Jonas completed the rout.
(Reporting by Richard Martin; Editing by Peter Rutherford)
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