Casillas, 33, was heavily criticised for his performances at the 2014 World Cup and is facing strong competition from David de Gea for the La Roja gloves.
However, Fabregas has said that the Real Madrid stopper, who has 156 international caps to his name, still has a lot to offer.
"To me, Iker seems committed and dedicated," Fabregas told AS.
"We all know that he went through a very delicate time at his club but here, with the national side, he looks happy and settled.
"For his character, he's the kind of player who will make his exit with dignity when he feels it's time to call it a day but that moment hasn't arrived yet. He seems fine to me."
Spain face Macedonia in their Euro 2016 opening qualifier this evening.