The Real Madrid keeper believes that the defender's injury is a big setback for La Roja, but insists that the team can go all the way in the tournament in Poland and Ukraine
The 34-year-old defender confirmed earlier on Wednesday that he will not be able to take part in the tournament after sustaining a knee injury which will keep him out of action for six weeks.
"I spoke with him on Tuesday, he is upset and sad, but understood that he had to take a break," the Real Madrid goalkeeper told reporters.
"I agree with coach Vicente del Bosque that this is a serious loss, not only on the field, but also when it comes to the group.
"He is one of the captains, together with Xavi and myself."
Casillas also commented on the situation of Barcelona striker David Villa, who remains a serious doubt for the competition after sustaining a fractured tibia at the Club World Cup in December.
"It would have been ideal if he could have played some minutes before the end of the season," he admitted.
Questioned about Spain's chances in Poland and Ukraine, the 30-year-old custodian admitted that La Roja can retain their Euro crown.
"We can win the tournament, although we face stiff competition," Casillas noted. "The biggest threats are Germany and Netherlands."
Spain kick off their Euro campaign against Italy on June 10.