The 32-year-old goalkeeper is refusing to doubt himself despite his continued absence from the starting XI at the Santiago Bernabeu and still hopes to win back his first-team place
The Spain international watched on from the bench last term following the arrival of Diego Lopez during Jose Mourinho's stint in charge and has continued to be overlooked since Carlo Ancelotti took over the reins this summer.
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"I think that believing in yourself is what is most important; never doubting yourself and never letting anyone else make you doubt yourself," the 32-year-old told Head and Shoulders as part of an advertising campaign.
“If people make you doubt yourself then you’ve had it. If you have done it before, why can’t you do it again."
Casillas was handed a recall for los Blancos' Champions League opener away to Galatasaray two weeks ago but a collision with Sergio Ramos's elbow saw him brought off injured just 14 minutes into the game.
Real Madrid are back in Champions League action on Wednesday as they host FC Copenhagen on the back of their 1-0 defeat to Atletico Madrid in La Liga on Saturday.