Real Sociedad have confirmed that Carlos Vela suffered a torn meniscus during Saturday's 4-1 defeat at Real Madrid in the league.
The Mexican started as the lone forward at the Bernabeu, but lasted just 17 minutes after picking up a knee injury.
Via an official statement, Sociedad have revealed the extent of the damage, but refused to put a timescale on the striker's recovery.
Various reports have suggested that Vela will be on the sidelines for at least two months.
The news will come as a big blow to David Moyes, who has been frustrated in his attempts to sign a new striker in the transfer window.
Vela has netted seven times for the Basque outfit this season.
