Madrid have slipped 11 points behind their unbeaten Catalan rivals to cast doubt over boss Jose Mourinho's future beyond the end of the season.
Mourinho, who has been booed by sections of the Bernabeu crowd in recent weeks, has responded by guiding his side to a crucial derby victory over Atletico Madrid last weekend before completing their progress to the last 16 of the Champions League in midweek.
Madrid's European duties are now on hold for two months and Casillas believes they must use that time to kickstart their flagging La Liga fortunes.
More specifically, he has targeted a hectic schedule of five games in 15 days - which includes both legs of their Copa del Rey last-16 tie with Celta Vigo - before the winter break as crucial to their hopes.
That run begins with a trip to surprise package Valladolid on Saturday and Casillas knows they can not afford to drop any points.
"We have targets set for the end of the year," the Spain goalkeeper told the club's official website. "It is perhaps a pity that the distance between us and Atletico in the league, despite having reduced it last weekend, and with Barcelona, is quite big.
"But we have made it through to the next round of the Champions League; between now and February there will be time to prepare.
"Now we want to focus on the league and cut the points difference with the teams above, and keep getting through the rounds in the Copa del Rey."
Speaking about tomorrow's match against Valladolid, who have won their past two to sit seventh, Casillas added: "It will be a difficult match. It is a tough ground to visit where the fans are enjoying the team playing in the first division again.
"Valladolid are playing some very good football and come into the match having beaten Sevilla convincingly.
"I think Saturday will be a tough match and they will make things very difficult for us."
Casillas is set to return to the starting XI after he was one of seven changes for the midweek 4-1 win over Dutch champions Ajax.
Mourinho was able to rest the majority of his defence in that game with Pepe, Sergio Ramos and Alvaro Arbeloa all sitting out.
The only regular, Fabio Coentrao, limped out in the first half with a hamstring problem and with Marcelo already sidelined, Mourinho will be forced into a reshuffle.
On-loan midfielder Michael Essien could provide a longer-term option after he returned to training this week.
Valladolid could be boosted by the returns of Manucho and Marc Valiente from injury and, after their good recent run, midfielder Oscar Gonzalez believes they are capable of springing an upset.
"Of course we can beat Real Madrid. We will compete as equals even though we know it will be difficult," he told the club's official website.
"It will be essential to defend very well, which we have been doing so far this season."