Speaking to reporters from outside the hospital in Porto after being discharged on Monday, Casillas paid tribute to the medical staff that had cared for him over the past week, and expressed his gratitude for the messages of support.
But he also stressed that he would take his time over making a decision about whether to return to football in the future.
“I will have to wait a couple of weeks or a couple of months, the truth is I don’t care (how long),” Casillas said.
“I don’t know what will happen in the future but the most important thing is to be here and to be able to speak with calm.”
A World Cup champion with Spain in 2010, Casillas spent 16 seasons with Real Madrid, winning three Champions League titles and five La Liga crowns.
The Spaniard, who turns 38 later this month, moved to Porto in 2015 where he won the Portuguese title last season.
Widely considered one of the greatest goalkeepers of all time, Casillas made his professional debut as a teenager for Real Madrid and went on to make 725 appearances for the club.
He was an integral part of the Spanish national team as it dominated international football for four years, captaining his country to victory at Euro 2008, the 2010 World Cup and Euro 2012.
Casillas signed a new two-year contract with Porto in March and has helped the club qualify for the knockout stages of the Champions League for the past two seasons.
He currently holds the record of most career Champions League appearances with 177.
CNN’s Matias Grez in London contributed to this report.