Real Sociedad have confirmed that midfielder Sergio Canales suffered a torn anterior cruciate ligament in his left knee in the first half of their match away to Real Madrid on Wednesday.
Canales had to be taken off on a stretcher on the stroke of half-time at the Santiago Bernabeu after Cristiano Ronaldo had put the hosts ahead from the penalty spot.
The former Madrid midfielder fell to the turf under no challenge as he attempted to pass the ball and appeared to be in immediate distress.
During the game's second half, Sociedad confirmed the news on their Twitter account.
"Further tests will be conducted to determine the diagnosis," the post read.
Sociedad also revealed that striker Imanol Agirretxe, who was also withdrawn before the interval, had to be taken off after suffering a sprained ankle in a challenge with Madrid goalkeeper Keylor Navas.