Spain entered the tournament as one of the favourites, but fell to consecutive 1-0 defeats against Group D opponents Japan and Honduras.
The 19-year-old, who returned from injury to start against the South Americans, insists he is unfazed by the criticism that was directed at him following his failure to seize several scoring opportunities that would have levelled the score.
"We all saw what happened during the match, but several elements remained out of our control," Muniain commented via his Twitter account.
"I am still proud of all my team-mates, we went out with our heads held high. I love this sport for the way it is an ongoing challenge. All I can say is we did our best.
"I just want to thank those of you who criticised me. It just makes my motivation increase."
Luis Milla's men will be playing for pride against Morocco in their final Group D fixture on Thursday.
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