Barcelona will be without captain Carles Puyol for up to eight weeks after he dislocated his elbow in Tuesday's Champions League win over Benfica.
The 34-year-old was stretchered off in obvious pain after he landed awkwardly when contesting a header in Lisbon.
Puyol, who has only just returned to action after a hamstring problem, was taken to hospital on his return to Barcelona, and scans confirmed the extent of the damage.
A Barca statement read: "The club captain will undergo conservative treatment and is expected to be out for about eight weeks."
Barcelona's defence is already depleted by Gerard Pique's foot injury, and they will now take on Real Madrid in Saturday's El Clasico without their skipper.
Coach Tito Vilanova said: "I've always said it, we've always had problems with our central defenders, but we'll move forward.
"It's not just that we've lost Puyol for the Clasico on Sunday, but we've lost him for other matches as well.
"The injuries mean we will have to play players in positions that aren't their own, but we'll use our youth players, that's why we have them."
Puyol's injury could also rule him out of the remainder of Barcelona's Champions League Group G fixtures, including the double header against Celtic.