Spain coach Luis de la Fuente fears that forward Yéremy Pino could miss the rest of the World Cup with a shoulder injury.
Pino came on as a substitute during the second half of their 1-0 victory on Friday in Guadalajara but got injured late in the game.
The player from Conference League winners Crystal Palace managed to complete the match but examinations are to determine the exact nature of the injury.
"Unfortunately it could be a collarbone injury. We will de la Fuente said post-match.
Fearing the worst because Pino was in big pain, he added: "He has an injury that could lead him miss the [remainder of the] World Cup."
De La Fuente praised Pino for completing the match despite the injury as Spain had already made their five allowed substitutions.
"I am very grateful to him, and his team-mates are even more so,” the coach said, naming Pino's attitude "heroic."
European champions Spain finished top of Group H with the victory and will play the second-placed team from Group L, either England, Ghana or Croatia, in the last 32 on Thursday.
