The Real Madrid stopper admitted he would concede the title to the club's main rivals if it meant winning in Europe, though assistant coach Aitor Karanka does not share that view.
A number of supporters and pundits have written off Jose Mourinho's side in the race for the Primera Liga title, after a poor start to the season has seen the club fall 11 points behind Barcelona.
But goalkeeper Casillas admitted that he would give the team's fierce rival a landslide victory in La Liga if Madrid was assured of the Champions League.
"I do not have to think about it. If you told me now that we would finish 25 points behind Barcelona in La Liga that is obviously not ideal as the domestic competition is very important, But if you tell me that we are going to win La Decima, I would sign for that right now," he told La Sexta.
Mourinho is currently preparing his side for its Copa del Rey round of 16 match against Celta Vigo on Wednesday, and assistant coach Aitor Karanka denied that the club has only one priority and that it will be doing all it can to win all the competitions they take part in.
"Is the priority the cup or the league? At this club, and for its coaching staff, the priorities are all titles and all matches," he told a press conference.
"The cup match has no less priority than any other match."
The former Athletic Bilbao defender also revealed that Los Blancos are doing all they can to close the gap at the top of the league as they aim to win back-to-back titles for the first time since 2008.
"We do not think like Casillas," the assistant coach told reporters. "The coaching staff are already quite uncomfortable with the gap and we do not want to be further behind. Here you fight for every title, and so we will continue addressing it."
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