The midfielder became the youngest player to score at a World Cup tournament since Brazil legend Pele in 1958.
Head coach Luis Enrique raised eyebrows when he called up Gavi for his first cap in October 2021 -- making him Spain's youngest ever international just two months after his Barca first team debut -- but his faith has since been justified.
"The most surprising thing for everyone is that someone, at 18 years and 100 days old, has that amount of self-confidence," Luis Enrique said in a news conference on Saturday.
"You have to look at the intelligence he has in his positioning, how he dominates the technical aspects of football, and his physical capacity despite being 18. We have to stay calm, he's 18, but he can define an era."
Gavi, who has already made 14 appearances for his country, scoring three goals, was awarded both Tuttosport's "Golden Boy" prize and France Football's Kopa Trophy for the best under-21 player in Europe last month.
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Germany go into Sunday's game at the Al Bayt stadium needing to win, after suffering a shock 2-1 defeat to Japan in their opening match.
"You look at their shirt and see four stars [for four World Cup wins]," Luis Enrique added. "I played against them [at the World Cup] in 1994 and their physical level was incredible... If anyone knows how to compete, it's Germany. Their history is there and they have world-class players."
By contrast, the challenge for Spain has been how to manage expectations after their record-breaking win over Costa Rica.
"You always hope for the best, but maybe I didn't expect such a potent version [of the team]," Luis Enrique said. "I'm better at dealing with problems. I'm a idiot like that."