The Spain goalkeeper picked up a leg muscle problem and could see his campaign in Brazil brought to a premature end
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 could miss the remainder of the group stage after suffering a gluteal injury and was forced to sit out of training on Monday.
De Gea underwent medical scans on 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 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 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 sit bottom of Group B, and will also take on Australia following their clash with Chile at the Maracana on Wednesday.