Diego Lopez has been the undisputed first choice in the league this season, while Casillas has been the stopper for the Copa del Rey and Champions League.
With the final of the European Cup to come against Atletico Madrid later this month, Ancelotti has confirmed that Casillas will feature in the league over the next couple of weeks.
"I've decided. Tomorrow Diego Lopez will play, then on Wednesday [against Real Valladolid], Iker Casillas. Then against Celta, Diego Lopez will play and against Espanyol, Iker Casillas," Ancelotti told reporters at a press conference.
"I've spoken with them and they're both in agreement with that. I've taken this decision because Iker Casillas needs to play during this month. I don't think it's fair to keep him out for a month, when he needs to prepare for the Champions League final."
Madrid host Valencia in the league tomorrow evening.