Real Sociedad midfielder Esteban Granero has torn a ligament and damaged the meniscus in his right knee and will require surgery, the Spanish club said after tests on Wednesday.
Granero suffered the injury and left the pitch on a stretcher during Sociedad's 2-0 defeat at home to Shakhtar Donetsk in Tuesday's Champions League Group A opener.
No date for the operation or a timescale on how long the player would be sidelined were given.
Granero joined the San Sebastian-based side from England's Queens Park Rangers last month, on a loan deal for the season with an option to make the move permanent next June.
Real Sociedad are seventh in La Liga and visit Bayer Leverkusen in their next Champions League outing on Oct. 2.