Athletic Bilbao have confirmed that midfielder Iker Muniain has ruptured the anterior cruciate ligament in his right knee.
The 24-year-old suffered the injury during Thursday's Europa League defeat to Zorya Luhansk.
Muniain spent more than eight months on the sidelines in 2015 after damaging ligaments in his left knee.
In a statement on their website, Athletic Bilbao said that Muniain will now see a specialist to determine how long he will be out for.
Lesión de Iker Muniain https://t.co/G6Clxm0Tjt #athletic
— Athletic Club (@AthleticClub) 29 September 2017