Cameroon striker Vincent Aboubakar struck his first of two goals from Alex Sandro’s deflected pass in the eighth minute. Maxi Pereira set him up in the 61st for a second goal.
Silvestre Varela netted in the 84th to secure the first three points of the campaign forJulen Lopetegui’s side.
Casillas left Madrid last month after 16 seasons, three Champions League trophies and five Spanish league titles, following a dip in form during the past two seasons.
The Spain captain had a placid night in front of his new fans at Porto’s Dragon Stadium, responding well with the few saves he had to make.
Elsewhere, defender Goncalo Brandao scored for both his Belenenses side and netted an own goal in a 3-3 draw with Rio Ave.
Benfica begins its league title defense at home against Estoril on Sunday.