Casillas' position in the Spain squad has been called into question ever since he made a series of high-profile errors at the 2014 World Cup.
His selection continues to draw criticism from sections of the Spanish media, but Barcelona full-back Jordi Alba leapt to the defence of his international team-mate.
"We know the quality of Iker and we are with him all the away," the 27-year-old told reporters ahead of Spain's friendly against Italy in Udine on Thursday.
"For me, he is one of the best three or four goalkeepers in the world. It's not only me who thinks this, either."
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Jordi Alba also reflected on the Euro 2012 final against Italy, where he scored in a 4-0 win as Spain claimed back-to-back European Championships.
"Everything went perfectly in that final," he added.
"But Italy are always difficult to beat. They're a team with a really strong spine."