Real Madrid lost 2-1 at Sevilla on Wednesday to fall behind Atlético Madrid and Barcelona in the race for the Spanish league title.
Cristiano Ronaldo put Real Madrid in charge with his league-leading 27th goal of the season in the 14th minute, but they relinquished the lead when Sevilla striker Carlos Bacca scored five minutes later, then notch a second goal in the 73rd minute to complete the comeback.
Diego Costa's second-half header gave Atlético a 1-0 victory at home over Granada as it moved three points clear of Madrid with eight games remaining.
Lionel Messi and Neymar helped Barcelona keep pace, one point behind Atlético, but the 3-0 win over Celta Vigo may prove to be very costly, as it came at the cost of goalkeeper Víctor Valdés, who twisted his knee during a clearance just 20 minutes into the match and had to be carried off the field on a stretcher.
The veteran keeper was taken to a clinic, where doctors determined he had fractured the anterior cruciate ligament in his right knee.
While Barça did not say how long Valdés will be out, the club said in a statement that he will likely need surgery and seems likely to miss the remaining matches of the La Liga season.
He may not be available to Spain for the 2014 World Cup, which starts in June.
Barça also remains in the chase for the Champions League title, gearing up for a first-leg quarterfinal clash with Atlético next week. José Manuel Pinto, 38, replaced Valdés in goal on Wednesday and made four creditable saves.
The injury comes at a critical time for both Barça and Valdés, who announced months ago that he planned to leave the club at the end of this season.
The Associated Press and EFE contributed to this report.