Casillas announced his retirement on Tuesday.
"Being a goalkeeper is a risky profession and [Casillas] always had that self-confidence to defend Real Madrid's goal," Hierro told Radio MARCA.
"He was very calm between the goalposts.
"When I joined the Spanish national team's sports management group, he told me that I was going to suffer what he had suffered when I was captain at Real Madrid.
"With me [as Spain's sporting director] he was an exemplary [Spain] captain.
"It hurts me that he didn't say goodbye on a field, but surely the world of football welcomes him with open arms."