Osasuna midfielder Sisi will have surgery on Wednesday after he injured the medial ligament in his knee against Rayo Vallecano.
The Rojillios beat Paco Jemez's side 1-0 at El Sadar yesterday evening, but the win was soured somewhat after the 26-year-old suffered what will be a long-term lay-off.
“We must operate,” Osasuna chief doctor Patxi Cipriain told reporters today. “The cruciate is not completely broken, but almost.
“The knee is unsustainable as it is, so we will have to operate.
“Like always with these sorts of injuries, he'll be out for between six to eight months.”
Sisi has been almost ever-present for Jose Luis Mendilibar's men this season, playing 10 League games.
