Eliminated from the King's Cup by third-tier Alcorcon in November, Real climbed above champions Barcelona on goal difference last weekend and play at struggling Real Valladolid in Sunday's late kickoff after Barca host third-placed Valencia.
Real's dream of winning a 10th European title in May's final at their Bernabeu stadium was shattered by visiting French side Olympique Lyon, who snatched a 1-1 draw to progress to the quarter-finals 2-1 on aggregate.
"We have to remain calm and think about the team," Spain captain Casillas told reporters after the defeat, the sixth straight season Real have failed to make the last eight of Europe's elite club competition.
"We can't let it hinder the good form we are on in the league," he added. "This championship is all we have left. We need to make a fresh start."
Barca, level on 62 points with Real, will almost certainly be without striker Zlatan Ibrahimovic for the Valencia clash, which pits top La Liga marksman Lionel Messi (19) against second-highest scorer David Villa (17).
Ibrahimovic was sent off in last weekend's 2-2 draw at Almeria and Barca are waiting on the result of an appeal against both the Swede's dismissal and the referee's decision to send coach Pep Guardiola off the bench.
If the appeal is rejected, Guardiola could bring in teenage striker Bojan Krkic or out-of-sorts France captain Thierry Henry as a straight swap or move Argentina forward Messi in from the right where he is typically deployed.
DIRECT RIVAL
Valencia, who play their Europa League last 16, first leg at home to Werder Bremen later on Thursday, are 15 points behind the top two in third, four ahead of Sevilla, who host sixth-placed Deportivo La Coruna on Saturday.
Sevilla's Brazilian striker Luis Fabiano should be available to coach Manolo Jimenez after he missed last weekend's 3-2 defeat at Real, and Mali forward Frederic Kanoute said on Wednesday he was getting back to full fitness.
"I have been a bit out of form this year but I am not worried because I know I can get back to my best level," Kanoute told a news conference.
"The (Deportivo) match is extremely important after last Saturday's disappointment," he added. "We are playing a direct rival and if we win we'll be more relaxed."
