The Italian is concerned about the condition of the players after the tournament in Qatar that kicks off on November 20.
"Until the World Cup we are not going to have many problems because the players want to get to the World Cup in good condition," Ancelotti told a news conference ahead of Real's LaLiga opener at Almeria on Sunday.
"The big question is how the players will return after the World Cup, ahead of what is the most important moment of the season for their clubs.
"We have to evaluate the tiredness. These are things that we have questions about today. It's a new experience for all of us."
Ancelotti said this was one of the main reasons he urged the club not to let any of the key players that won the Champions League-LaLiga double leave in the transfer window and why he wanted to strengthen some positions so he has a deeper squad.
"The idea is to rotate more than last year and I'll be able to do it more this season because we have a very strong squad, more complete in every level than last year," Ancelotti said.
"We have to keep everyone motivated and there is a World Cup that means we have to have a fairly big squad and in good condition."
Ancelotti said he expected to use forward Eden Hazard (31) more as a jack of all trades this season, but the Belgium international needed to be fully fit to feature.
"I still don't know how much he can play, but he definitely will play more than last year," the coach said.
"He can play in different positions, on the wing, on the left, on the right, we are testing him as a false-9... It's a position in which he can also demonstrate his quality."
Ancelotti did not reveal his starting eleven for Sunday, but he will be without Rodrygo who has a leg muscle problem.