Athletic Bilbao striker Guillermo could be out of action for the next month after injuring his hamstring in Sunday's goalless draw with Valencia.
The 21-year-old, who has been given a run in the side due to an injury to Aritz Aduriz, suffered the problem in the first period of the La Liga clash at the Mestalla.
According to Inside Spanish Football, a medical examination revealed that the Spaniard had suffered a hamstring injury, but a return date has not yet been set.
Bilbao have taken 12 points from their 11 games this season, with that total leaving last season's fourth-place finishers in 11th spot in the table.
