Casillas was targeted by some sections of the Bernabeu during Madrid's 2-1 home defeat to Atletico Madrid at the weekend.
The goalkeeper was in particular blamed for Atletico's first goal, when Tiago headed home from close range.
However, Cesar, who played for Los Blancos between 2000 and 2005, believes that the criticism is "very unjust".
"It wasn't Casillas's fault because it was a very difficult ball. We are in the middle of something here where people look very closely at the goalkeeper," he told COPE.
"We must not be unfair. The fans should support those that play every weekend. I don't know if there is a solution. It's happened before on other occasions and it always seems very unjust. It's no good for Madrid if they are playing under pressure from their own supporters. They should be supporting the team in difficult times."
Madrid have lost two of their first three league games this season.