The 18-year-old is yet to make his first appearance for the capital giants, as he underwent a knee operation in August and then picked up a thigh issue in the latter stages of his recovery.
Guler has been back in training in recent weeks and was named in the squad for the team's last two matches against Rayo Vallecano and Braga in La Liga and the Champions League respectively.
After not appearing against Braga, Ancelotti said that the club were "being very careful" with the attacker, revealing that he would only feature "when he's 100%.
Speaking ahead of Saturday's La Liga game with Valencia, the Real Madrid boss revealed that the youngster had suffered a fresh issue during a training session ahead of the Braga contest, and journalist Guillermo Rai has suggested that he could be missing for a month.
"Unfortunately it has happened before the Braga game. It seemed like nothing, but it's still bothering him. It's not a serious thing," said the Italian during his press conference on Friday.
"The player is depressed because he wants to play. It's a small setback, it's not a relapse of the old injury and we have the whole break to get him back better.
"The first injury was a meniscus tear that he had last year. He was operated on and, as often happens, it changes your position with the other leg and it can happen that during recovery you can have muscular problems. That's what happened to him.
"His future is beautiful, beautiful. He is a very talented player. I can understand that the player is disappointed, but it's just a muscular problem.
"I have personal experience that I stopped for two years when I was 21 and then I came back. Guler's future is not in doubt. It's just a moment and I hope he can recover after the break.
"It's quite normal for a player to be upset. He is new here, he wants to contribute to the team.
"It's always difficult for an injured player to cope with the moment because he see his teammates training and playing. His sadness is quite normal, but we give him all the love in the world. We don't rush him. His future is here."
There was a host of speculation surrounding Guler's future over the summer when it became clear that he would be leaving Fenerbahce, but Real Madrid ultimately won the race for his signature.
The forward managed nine goals and 12 assists in 51 outings for Fenerbahce in all competitions, while he has one goal in four appearances for Turkey.