Spain beat Uruguay 1-0 to finish top of Group H, but the win brought an injury scare over Nico Williams.
The winger went down with a fresh muscle problem after just 15 minutes on the pitch, despite Spain already taking a cautious approach with him after an injury-hit season at Athletic Bilbao.
Head coach Luis de la Fuente said Williams felt discomfort and that the issue could be a strain or simply fatigue. He added that Spain would wait until the following day for a clearer assessment.
The concern is added by the possible loss of Yeremi Pino, who may miss the rest of the World Cup because of a collarbone injury. Lamine Yamal has only just returned from injury, while new Liverpool signing Victor Munoz has not yet featured in the tournament.
That leaves Spain waiting on the fitness of key wide players at a crucial stage of the competition.
