Barcelona and Madrid will have plenty of game time against each other ahead of the Champions League semi-finals as they meet this weekend in La Liga before clashing again in the Copa del Rey final next Wednesday.
Madrid have eight points to make up on Barca at the top of the league table, and goalkeeper Casillas insists that must be their immediate priority.
"We must now focus on our league clash against them because it is the nearest of them all," he said.
"It will be completely different to our fixtures in the semi-finals.
"I am Real Madrid's captain, and I would be disrespecting Madridistas if I said Saturday's Clasico didn't matter.
"We want to defeat Barcelona on Saturday to earn more points because the league title is still attainable. It isn't all lost."