Assistant coach Aitor Karanka revealed to reporters that "he is now fit to play" but it is unknown whether he will go straight back into the starting lineup.
"I don't have to evaluate if he is training well or badly, I can only say that he is important to us," Karanka added.
"Iker Casillas trained quite well at Real Madrid," he told radio station COPE. "Having Iker with us again is very reassuring.
"Yesterday he played an 11-a-side match and we are happy that he has returned."
The match kicks off on Saturday at 5pm.