The Italian is yet to name who will start in goal for Los Blancos in Sunday's clash against Real Betis, but revealed Daniel Carvajal could play ahead of Alvaro Arbeloa
The Blancos begin their 2013-14 campaign against Betis and captain Casillas is widely expected to return having been ousted from the side by Lopez last season under former coach Jose Mourinho.
"I still haven't decided who will play in goal, I will do tomorrow," Ancelotti told reporters on Saturday ahead of his first competitive game in charge of the Spanish giants.
"I'm not going to announce my starting line-up because I still haven't spoken with the players, I prefer to speak with them first and then we'll put out a competitive team.
"Diego Lopez began work on July 15 and Casillas on August 1. That's not a big difference for a goalkeeper. Both players had good pre-season campaigns and played well - Iker did well against Chelsea, Diego against Inter.
"It's not an easy decision for me but it's normal. As a coach, choosing which players should play is the hardest part of my job."
Although he was tight-lipped regarding who would be his No.1 goalkeeper at the Santiago Bernabeu in their Liga opener, the Italian hinted that Daniel Carvajal is set to start at right-back ahead of Alvaro Arbeloa.
"Arbeloa has just come back from a long trip [Spain travelled to face Ecuador on Wednesday] and he'll be a bit tired, Dani has trained well this week and he has a small advantage," he added.
On the injury front, Madrid are still set to be without veteran midfielder Xabi Alonso, new signing Asier Illarramendi and centre-back Raphael Varane, though Pepe is fit to partner Sergio Ramos in defence.
"Xabi Alonso is close to playing," Ancelotti continued. "The same goes for Illarramendi. Pepe got a knock while playing with Portugal but he trained well and he'll play tomorrow. Varane is still training by himself and following a specific programme, it'll be another few days before he can play again."