A smiling Casillas addressed reporters outside the CUF Hospital in Porto and admitted he was uncertain about what the future holds.
"I do not know what the future will be, but the most important thing is to be here, to be able to speak and transmit to everyone how I am and how I feel," the World Cup winner said, according to BBC Sport.
Casillas added that he would now rest "for a couple of weeks or a couple of months."
"A few days ago I went through a difficult situation, something that can happen at any moment of life, but fortunately I'm fine. I am grateful and feel that I am very lucky," he said.
Casillas, who is in his fourth season in Portugal, made 725 appearances for Real Madrid during a decorated 16-year career which included five Champions League titles and three La Liga crowns. The 37-year-old is Spain's most capped international on 167, with his last outing for La Roja coming in 2016.