The 24-year-old went down clutching his left knee under no pressure late in the first-half, having already had two operations on his right knee after suffering cruciate ligament damage when at Valencia.
"Medical tests carried out are compatible with a torn anterior cruciate ligament," Sociedad posted on their official Twitter account.
"He has responded twice before and I am convinced that he is going to fight to return to enjoying football, which is what he loves," added Sociedad coach Eusebio Sacristan.