The Spain skipper suffered a broken metacarpal bone after being accidentaly kicked by Alvaro Arbeloa during los Blancos' Copa del Rey quarterfinal second-leg clash with Valencia.
Casillas underwent surgery on the injury early on Friday afternoon and, although the procedure went according to plan, Dr Manuel del Cerro has confirmed the 31-year-old faces between eight and 12 weeks out of action.
"The operation went very well," he told reporters. "The broken fragments have been reduced and two screws fitted. He has had a total reconstruction. The screws will stay inside. They are 17 milimeters long and two millimeters wide.
"The recovery period is eight to 12 weeks. It is better to wait and not talk about deadlines.
"It was much better than I expected. He [Casillas] is OK, but a little sleepy. Now he will spend three weeks in a cast, and we will go step by step so that he can recover without any consequences."