Jose Mourinho’s men booked their place in the quarter-finals of the cup with a 1-0 victory over Malaga at La Rosaleda on Tuesday night to progress as 4-2 winners on aggregate.
Madrid are now almost certain to face Barca in the last eight, with their great rivals taking a four-goal advantage into the second leg of their tie against Osasuna on Thursday night.
However, although the hype over yet another Clasico double-header has started already, Casillas is trying to remain focused on Sunday’s trip to Mallorca.
“Before worrying about a possible tie with Barca we have to defend the advantage we have at the top of La Liga,” the goalkeeper is quoted as saying by Marca.
Casillas also pointed out that it is disrespectful to Osasuna to start talking about facing Barca, especially in light of the fact that Mallorca booked their place in the quarter-finals with a 6-3 aggregate win over Real Sociedad, having lost the first leg 2-0.
"In the dressing room we have discussed what can happen in football and we've all seen it in the Mallorca game,” the Spain international said.
Should Barca finish the job against Madrid, they will travel to the Santiago Bernabeu on January 18 before hosting Los Blancos at Camp Nou seven days later.
Starting in November and continuing through January, Goal.com is publishing exclusive extracts from Graham Hunter's highly-anticipated new book, 'Barca: The Making of the Greatest Team in the World'. On Friday, the latest extract will take a closer look at the exciting emergence of Fifa Ballon d'Or winner Lionel Messi at Camp Nou: Don't miss it!