Carlo Ancelotti believes the biggest unknown surrounding Real Madrid's potential defence of the Champions League is the prospective physical condition of his players when they return from the World Cup.

Madrid begin their quest for a 15th European Cup on Tuesday when they visit Scottish champions Celtic.

The group stage is condensed this season because of the World Cup, moved from its usual Northern Hemisphere summer slot to November because of the extreme heat in Qatar.

Teams involved in the group stage will play six games in under two months, with the battle for a place in the last 16 coming to a close on November 2.

On top of domestic commitments, that makes for a crowded fixture list. Yet Ancelotti is less concerned with the effects of the schedule than he is about the potential toll on his squad of playing in a mid-season World Cup.

"Until the World Cup, I think we won't have any problems," Ancelotti said of the unusually busy fixture list. 

"The players are prepared and motivated. The question is how the World Cup players will return. It cannot be known."

Madrid are not seen as favourites for the Champions League despite their remarkable run last season. Los Blancos delivered thrilling comebacks against Paris Saint-Germain, Chelsea and Manchester City before beating Liverpool 1-0 in the final.

City are the bookmakers' favourites ahead of PSG, Liverpool and Bayern Munich. Ancelotti, though, is happy his team are not in the top four in that sense.

"I'm not surprised. I see it as a good sign," he added. "I think that last year we had even less probability in the statistics. So we have improved. 

"What happened last year will happen. We are going to compete until the end and hopefully we will compete in the final."

Ancelotti's team are, however, overwhelming favourites to win Group F, which also features RB Leipzig and Shakhtar Donetsk, as well as Celtic.

But Ancelotti pointed to Madrid's shock home defeat to Sheriff Tiraspol in last season's group stage when asked about their seemingly easy draw.

"It is not an easy group, because we know the teams very well," said Ancelotti. "Celtic are doing very well, we have to respect them, like Leipzig and Shakhtar. 

"In the group stage, there are surprises, especially in the first half. We have a pretty vivid memory of last season with Sherriff at home. We respect the group and I think it will be an even group."