The Spain goalkeeper picked up a leg muscle problem and could see his campaign in Brazil brought to a premature end.
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The Manchester United No. 1 had been tipped by some to make a starting appearance for Vicente del Bosque's men against Chile in Rio de Janeiro on Wednesday following Iker Casillas' horror show in the 5-1 loss to Netherlands.
But now the former Atletico Madrid man was forced to sit out of training Sunday after suffering a gluteal injury and could miss the remainder of the group stage.
De Gea underwent medical scans Sunday, and it is feared that a significant strain will end his tournament, with initial reports in Spain suggesting he will be out for at least 10 days. Medical staff members have confirmed they will make day-to-day assessments of the 23-year-old's condition.
Casillas' poor showing in Salvador on Friday led to the Spain first-choice keeper accepting culpability for the loss in the media, and some have questioned his place in the side. However, if De Gea is forced to leave the camp as a result of his injury, only Pepe Reina remains at Del Bosque's disposal as a potential replacement.
Spain currently sits at the bottom of Group B and will also take on Australia following its clash with Chile at the Maracana on Wednesday.