Iker Muniain suffered ligament damage in his right knee, Athletic Club confirmed after tests on the player on Friday.
The forward picked up the injury in Thursday's Europea League game against Zorya and is set for at least six months out of action.
Athletic's statement added that the player is "waiting for an appointment with the specialist to set a date for surgery."
Muniain, still just 24, knew as soon as he went down that the injury was serious and he was carried off on a stretcher in tears.
It's the second time Muniain's suffered a similar injury in his short career. In April of 2015 he also damaged the ligaments in his left knee in a game against Sevilla.
On that occasion, he did no reappear until December 20 of the same year, more than eight months after he picked up the injury.