The Spain captain broke a bone in his thumb during the Copa del Rey match with Valencia and is expected to be out for six-to-eight weeks of the knock, which the 33-year-old feels could prove costly to the champions.
"It's one more problem for them, he is the heart and soul of Real Madrid and their captain. He's a huge loss for Madrid and for Spain,” he told reporters.
The Blaugrana midfielder also spoke of his shock at Jose Mourinho's decision to drop Casillas for the Liga matches against Malaga and Real Sociedad either side of the winter break.
"I speak with him a lot, we send each other messages. I sent him my best wishes as soon as he was left out of the Madrid team; I was taken by surprise by that.
"It was a shock to me that a player who has spent so many years as the No.1 and who was always in the team could suddenly be left out, but that's their problem.
“It's a complicated situation, and it's not for me to give my opinion on. I read the news and I can see that the club is not in the best shape at an institutional level.
"But they will be very difficult opponents in the [Copa del Rey] semi-finals and it will be very for us to beat them."