Critics accused De Gea of an underwhelming display in the first two games of this year's competition, which saw Spain draw 3-3 with Portugal followed by a nervy 1-0 victory over Iran on Wednesday night, reports Efe.
"I think that (criticism) is normal when the bar is set too high, because people expect maximum effort, but what is truly important is what comes next," Casillas, a long-time Real Madrid stalwart who now plies his trade with Porto, said following the inauguration of the Football Park in Moscow's Red Square.
In his opinion, what was important for Spain in the group stages was simply to ensure its ascent to the knockout rounds, and once that goal was achieved, no one would remember any apparent mistakes De Gea made.
The Manchester United keeper was singled out by Spain's sporting media for failing to block a powerful long-range shot from Portugal star striker Cristiano Ronaldo in the first game.
Casillas said he was optimistic Spain could reach the final, which is to be played at the Luzhniki Stadium in Moscow on July 15.
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