Real occupied top spot in the league standings but lost ground in the title race after back-to-back defeats to Barcelona and Sevilla, though they bounced back with a crushing 5-0 victory over Rayo Vallecano last time out.
The 32-time La Liga champions are now three points adrift of league-leading Madrid rivals Atletico and Casillas knows the club can ill-afford to drop points against Real Sociedad on Saturday.
"We cannot afford anymore setbacks," Casillas told reporters.
"Tomorrow's game against Real Sociedad is very important for us. It'll be a hard outing and we need to get the points."
Cristiano Ronaldo, who has scored 28 goals in La Liga this season, will be rested for the match as he overcomes a knee problem.
And Casillas is also unlikely to play against Sociedad, with coach Carlo Ancelotti preferring Diego Lopez in Real's La Liga fixtures this season.
However, the Spain captain says he backs Ancelotti's decision.
"Carlo Ancelotti has always done the same since the start of the season," said Casillas, who has been linked with a move away from the Bernabeu.
"He trusted Diego Lopez for La Liga games, and he trusted me for Copa and Champions fixtures. It would have no sense to change that now.
"Diego deserves the respect and support and if he's a regular in La Liga since it started he should finish it. As simple as that."
Meanwhile, Casillas has voiced his displeasure with international team-mate Sergio Busquets following an incident during the 'El Clasico' last month.
Camera footage showed the Barcelona midfielder stamping on the face of Real Madrid defender Pepe.
Busquets stressed the movement was not intentional, while Pepe has since labelled him a 'liar'.
Now, Casillas has joined the argument.
"The image speaks for itself. Pepe has the full backing of the whole side," he said.
"I'm not going to go into more detail. It was a heated moment. Busquets, from my point of view, behaved poorly.
"When I see Busquets I'll give him a slap."
