Real Madrid midfielder Mahamadou Diarra has been sidelined for six to nine months, after undergoing arthroscopic knee surgery on Monday.
A brief statement on the club's official website said the Mali international underwent surgery on a "very serious knee injury that is likely to keep him out for six to nine months".
The 27-year-old injured his knee in October and briefly returned to action, but was consigned to the injured list when he began feeling sharp pain in his right knee at the end of last month.
Diarra, who moved to Spain from French team Lyon in 2006, is one of Real's most influential players.
His absence is another major blow for the nine-time European champions, whose Dutch striker Ruud van Nistelrooy has been ruled out for most of the season through injury.
His fellow Dutchman Royston Drenthe could also join the club's injured list, after the midfielder pulled out of training with a knock on Monday.
After their UEFA Champions League fixture against Zenit on Wednesday, Real face arch-rivals and La Liga leaders Barcelona at the Nou Camp on Saturday.