Courtois has missed his side's last four matches with a back problem, but it had initially been thought that the Belgium international would return for Sunday's clash with Barcelona.
The 30-year-old has ultimately been left out of the squad for the La Liga contest, with Ancelotti revealing that the stopper only just missed out on Sunday's fixture.
"Courtois is doing quite well, but he has trained little and he is ruled out for this game. The important thing is that he has recovered from his problem and now he just needs training sessions," said the Italian.
The Belgian could therefore be in line to make his return against Elche on Wednesday evening, although it seems more likely that he will be back against Sevilla next weekend.
Antonio Rudiger also emerged as a major doubt for the clash with Barcelona when he picked up a nasty cut to his head during the Champions League clash with Shakhtar Donetsk last week.
However, the Germany international has been named in the squad, and he will wear a mask if he takes to the field against Xavi's side.
"Rudiger is good and the mask is just to stop the wound from opening again. But, he is a warrior and he is excited. He brings a great mentality," the head coach added.
"Rudiger brings strength in duels, heading in both areas. It is not a surprise that he scored at the end. Like [Eder] Militao, they have this quality of the header. What he brings more are his mentality and his personality. A winner who never stops in training."
The 29-year-old has made eight La Liga appearances for Real Madrid this season, but Militao and David Alaba are expected to start in the middle of the defence.
Ancelotti also spoke about Karim Benzema during his press conference, with the France international finding it difficult to show his best form early in the campaign.
The 34-year-old has managed four goals and one assist in nine appearances, but he is yet to find the back of the net in three Champions League appearances in 2022-23.
Benzema was also missing against Getafe in the league last weekend through injury, but Ancelotti has said that the striker is in good shape for El Clasico.
"I've seen him much better this week, his condition has improved a lot. I'm convinced that tomorrow we'll see the usual Karim," he added.
Benzema has failed to find the back of the net in La Liga since a double against Espanyol at the end of August. body check tags ::