Alvaro Morata suffered an apparent hamstring injury while playing in an international friendly for Spain against Belgium on Thursday.
Morata lasted just 24 minutes at the King Baudouin Stadium before limping off and being replaced by Chelsea forward Diego Costa.
The 23-year-old Real Madrid striker went down off the ball and Spain's medical staff immediately signalled that he would have to be substituted.
Morata could be a doubt for Spain's opening World Cup qualifying match against Liechtenstein on Monday.
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