Much has been made of a potential rivalry between the two Real Madrid goalkeepers, with Lopez replacing Casillas as number one choice in La Liga fixtures, and the Spain captain featuring in the Copa del Rey and UEFA Champions League.
Real currently top La Liga by three points from city rivals Atletico Madrid with just 10 games to go in the campaign.
However, going against those reports, Casillas told Marca of Lopez last week: "So far, the coach has opted for Diego Lopez in La Liga and we are leaders thanks in no small part to his efforts."
"Iker is the team's captain and figurehead," Lopez said.
"I'm grateful for his comments. He's always had and will always have my utmost respect.
"We both have our own skill sets and we compete for a place in the best team in the world.
"It's a privilege (playing for Madrid). I defend the club's colours.
"I fight to play every game and we're all working hard to play as a unit."
He added: "It's going to be a crucial game, we know how important it is and we're looking forward to Sunday.
"They'll also see it as a great opportunity, but we have to defend our lead and take it one game at a time."
