Athletic Bilbao winger Iker Muniain faces a long spell on the sidelines after suffering a serious knee injury in the 1-0 Europa League defeat to Zorya on Thursday.
Tests confirmed that Muniain tore his right anterior cruciate ligament late in the game, and the 24-year-old will now undergo surgery.
A club statement said: "Tests have detected an ACL tear in Iker Muniain's right knee."
It is the second serious injury Muniain has suffered in his career after tearing the ACL in his left knee in April 2015.
Muniain, who won the European Under-21 Championship with Spain in 2011 and in 2013, has one senior international cap.