The Spain captain went under the knife during an hour-long operation in Madrid on Friday after suffering the injury during Wednesday's Copa del Rey quarter-final clash with Valencia.
Doctor Miguel del Cerro, who performed the operation, said in AS: "The operation has gone very well. He will be out for between eight to 12 weeks."
Madrid medical chief Carlos Diez said in a statement on his club's website: "Everything has been phenomenal. Two screws were put in and the hand has been perfect. It has been a complete success from a surgical point of view."
Casillas will now miss the Copa del Rey semi-final double-header against Barcelona, the Champions League last-16 tie with Manchester United and a host of Primera Division matches.
Antonio Adan, who has been controversially picked by coach Jose Mourinho ahead of Casillas in a handful of recent matches, is the frontrunner to replace the world and European champion although Madrid are reportedly in the market for a new goalkeeper to cover Casillas' loss.