Mourinho, who dropped Casillas from the Real side during his final season in charge in 2012-13 and was widely reported to have fallen out with the club captain, said he would not discard Casillas just because of one poor showing.
Holders Spain suffered a 5-1 loss against the Dutch in Salvador, their worst drubbing at a World Cup finals in more than 60 years, and need a win against Chile on Wednesday to get their bid for a second straight title back on track.
"The goalkeeper position is a very specific one and I wouldn't like to make a change just because of one poor game," Mourinho told Yahoo! Sports.
"I would only change something after a bad run of form that clearly shows a keeper lacks confidence and stability and his body language suggests it would be better to change," added the Portuguese, who is in his second spell at English club Chelsea.
"Iker has had a fine run with the Spanish national team and he has the confidence of his coach. One bad game is not sufficient reason to change keepers."
Spain coach Vicente del Bosque is unlikely to drop Casillas, whose performances in helping Spain win Euro 2008, the 2010 World Cup and Euro 2012 earned him the nickname "Saint Iker" among fans of the Iberian nation.
Del Bosque's options are in any case limited as one of his two reserve keepers, David De Gea, is injured. Pepe Reina is the other stopper in the squad.
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