The Wales international has not started for the Liga leaders since October's 5-0 win over Athletic Bilbao due to a gluteal muscle injury.
After a substitute appearance against Liverpool in the UEFA Champions league on Tuesday, the 25-year-old will return to the starting side as Ancelotti rotates his squad.
"Isco and James [Rodriguez] have played well through this period," he told a media conference on Friday.
"One of them will have a rest to put in a fresh player like Bale.
"I will take someone out. This rotation is something normal."
Ancelotti expects a tough test at the Santiago Bernabeu on Saturday against the team from Vallecas, noted for their attacking flair, and warns they must respect Rayo to avoid an upset.
"We have to respect a team who have a philosophy and apply that philosophy against every team," he added.
"They play football against every team and fight for survival. They have a very clear plan to play attacking football and it's a good plan and a positive one."
The Italian also confirmed Jese Rodriguez had begun the next stage of his recovery from a knee ligament injury, the 21-year-old returning to training on Friday for the first time, with Ancelotti hopeful he will be ready to play next month.
