The injury occurred during a scramble in the Madrid box when Álvaro Arbeloa accidentally kicked Casillas hand or wrist as he tried to clear the ball from danger. Casillas went down immediately, clearly in pain, and the team physio's raced on the pitch to check on the health of the keeper.
After several minutes of examination of his hand and wrist by the physio, Casillas shook his head and walked off the pitch in frustration. Antonio Adan entered the match in his place.
It's way to early to tell the extent of the injury but any type of issue with a goalkeepers hand or wrist potential bad news. Madrid will likely be hoping it's something minor like a sprain or bone bruise, but given the strength of the impact from Arbeola's kick, the possibility of a fracture or break is going to be a concern.