The Spain international goalkeeper has been replaced as Los Merengues' number one by Diego Lopez, having originally been dropped last term by Jose Mourinho.
New trainer Carlo Ancelotti has also chosen Lopez over Casillas and the Madrid idol admits that playing once a fortnight is causing him problems.
"It is not easy to play games like this when there's so much time that you are not playing," he said after the game against Juve.
"You notice that you are lacking certain things when there are two weeks between matches."
Casillas has found life as Madrid's second-choice stopper tough to adapt to and thinks that not playing often means it is difficult to convince Ancelotti to pick him.
But he boosted his claims with an inspired performance in his 133rd Champions League appearance, surpassing Raul's record for the club.
"It is hard to get into the team, so I am happy [with the display against Juventus]," he continued.
"You have to be at your best when you get the opportunity, so that the coach can see that he can trust you.
"It is not easy because you play every two weeks and it is difficult to keep your focus, but with everyone helping me out it is easier to deal with it well."