Spain goalkeeper David de Gea sat out training on Sunday due to injury, the reigning world champions have announced.
According to a medical statement from the Spanish football federation (RFEF), De Gea is struggling with a gluteal muscle problem and will be monitored day by day to see when he will be able to resume training.
The RFEF has not given a time-frame for how long the Manchester United 'keeper might be out for.
A statement read: "After undergoing tests it was decided that David de Gea would not take part in the team's afternoon training session owing to muscle discomfort in the gluteal region.
"The Spanish national team's medical services will evaluate day by day the goalkeeper's recovery to determine his reincorporation into the group."
The 23-year-old would have been an option to replace captain Iker Casillas if Spain coach Vicente del Bosque decides to ring the changes following his side's 5-1 defeat by the Netherlands in their Group B opener.
Pepe Reina is the other goalkeeper available to Del Bosque, whose side face Chile on Wednesday before taking on Australia the following Monday.