The Spanish champions booked their place in the Spanish Cup final with a complete performance that restricted Barça to very little going forward and contained their usual lightning speed on the counter-attack.
But Casillas believes he knows many of his international colleagues in the Barça side too well to think they will take defeat lying down.
"I know many of the Barcelona players and I know that they are competitive," he told the club's website.
Despite Real Madrid's joy in the cup, in the league it is Barça who have been the dominant force this season as they arrive in the capital with a 16-point advantage over Real.
And with another huge Champions League game to come against Manchester United on Tuesday, many expect Jose Mourinho to make a number of changes.
However, Casillas believes it is important to try and maintain their positive momentum heading into their trip to Old Trafford with the teams locked 1-1 after the first leg of their last-16 duel.
"It is a considerable lead, but with the crowd behind us we want to end a week that could be the turning point."
Casillas will definitely miss the match as he continues his recovery from a broken bone in his hand and Angel Di María is suspended after being sent-off in the 2-1 win at Deportivo La Coruña last weekend.
Barça meanwhile are recovering not only from the blow inflicted by Madrid in midweek but from their 2-0 defeat to AC Milan in the Champions League six days previously and midfielder Andrés Iniesta accepts they have some issue to address.
"The team is coming from some very good years and we deserve to be treated calmly when we lose to a great team like Real Madrid in the semi-finals of the cup," he told reporters.
"We need to think a lot about these two games, in what we have done well and what can improve. We need to recover the sensation of being a compact and infallible team."
However, the bad news didn't end on the field for Barca on Tuesday as Lionel Messi missed training on Wednesday morning and has been treated for fever like symptoms.
With Barca's Champions League return with Milan not until March 12 there is not the same pressure on assistant manager Jordi Roura to rotate his side, but even if Messi is fit there have been widespread calls for David Villa to be handed a start with Cesc Fabregas the most likely candidate to drop out.