Theo Walcott is hoping to make a rare appearance for Everton this season in Wednesday night's home Carabao Cup tie against West Ham.
The Toffees, who extended their 100 per cent Premier League start with victory at Crystal Palace on Saturday, are bidding to reach the last eight.
Walcott's only appearance this campaign was in the second round of the cup competition against Salford.
But he was an unused substitute in last week's third-round win at Fleetwood and did not make the bench at Palace last weekend.
"He was out because the rule is only to play 18 players and he was out of the squad," Ancelotti said.
Ancelotti said it had been a reluctant decision to leave Walcott out and does not expect the former England winger to leave before the transfer window closes next month.
"No, I don't think so," Ancelotti said. "The problem is the competition is really high now in this moment.
"Sometimes it can happen that a player like Theo is not involved."
Walcott has slipped down the pecking order after the high-profile arrivals earlier this month of James Rodriguez, Abdoulaye Doucoure and Allan.
"Unfortunately this is a decision I have to take and I'm not absolutely happy to take this kind of decision to leave out a player like Walcott, who is a professional in training and always ready," Ancelotti said.
Ancelotti has confirmed he is still hoping to sign a central defender on loan before the European transfer deadline on October 5.
"We are going to have a look and we will see," he said.
Ancelotti made six changes for the third-round win at Fleetwood and could rest the likes of Rodriguez, Doucoure, Allan and skipper Seamus Coleman again.
Coleman won high praise from Ancelotti after Saturday's 2-1 win at Selhurst Park, which left Everton third in the table, behind early leaders Leicester on goal difference.
"He's doing really well," said the former AC Milan, Chelsea and Real Madrid boss. "I think that Seamus is an example for all the others and for the new players of how to be a player on the pitch who produces the spirit.
"But not only on the pitch. Every day. Seamus is a great example of a captain. I had a lot of captains in my career. Seamus has a spirit. He is really tough.
"He's a great captain. I cannot make a list because I would forget someone. But like (Paolo) Maldini, John Terry, Sergio Ramos, he is that kind of captain."