Real go into the second El Clasico of the season four points clear of holders Barca at the top of La Liga.
Benzema was taken off with a leg injury in their win over Malaga at the weekend and will miss their UEFA Champions League last 16 second leg tie with Schalke.
With Real 6-1 ahead from the first leg, Ancelotti will take no risks with the France striker but expects the 26-year-old to be fit to face the Catalan giants.
"We will make a few changes (for Schalke) because some players are physically tired," he said.
"Benzema can't play tomorrow but we are confident he will be fit for Sunday. Jese, (Cristiano) Ronaldo, and (Alvaro) Morata will all start tomorrow
"Benzema is a very important player. We are lucky to be able to replace him when he is injured."
Provided they see out the tie as expected, Real will move a step closer to clinching the treble in Ancelotti's debut season.
The Italian stated Real are the most complete squad he has worked with as they seek to win La Liga, the Copa del Rey and Champions League titles.
"I've coached many teams with a lot of quality but it's hard to say which is the best," he continued.
"This squad is the most complete. It has character, experience and youth. I'm doing my best and I'm very happy here.
"We have a small advantage in La Liga at the moment but it's not the same in the Copa del Rey or Champions League.
"We have to play well (against Schalke) because it's a very important match. We have to be consistent. Playing well is the best way to prepare for the Clasico."
