The Italian will divide the remaining league games between the pair, while revealing he is wary of a backlash from Valencia on Sunday following their Europa League heartbreak
The Spain goalkeeper has only been used in cup matches this season, with Diego Lopez preferred for league fixtures.
But in preparation for their May 24 Champions League showdown against city rivals Atletico, Casillas will be used for the games against Real Valladolid and Celta Vigo.
Head coach Ancelotti revealed on Saturday: "Diego Lopez will play against Valencia and Celta (Vigo), Casillas against Valladolid and Espanyol.
"Casillas needs to prepare for the Champions League final during this month."
Ancelotti's side host Valencia on Sunday and the Real coach is wary of their scorned opponents after Stephane M'Bia's stoppage-time goal sent Sevilla into the Europa League final at their expense on Thursday.
Valencia trailed 2-0 after the first leg but moved into a 3-2 aggregate lead, only to be denied by M'Bia's late goal.
"Losing a chance to play in a final like what happened to Valencia is not easy," he added.
"We have to be careful. We need to win."