Mourinho stirred up controversy when he dropped Spain's number one and played reserve keeper Antonio Adan for the last La Liga game before the mid-season break away at Malaga in December, when the champions slipped to a 3-2 defeat.
"To be permanently in the comfort zone isn't good for players," Mourinho told a news conference ahead of their return to action on Sunday at home to the Basque club.
"All positions on the pitch are about confidence, it is fundamental for all the players, not only for the keepers.
"This translates into performance, but just as confidence is important so is competition, and we will try and strike that balance."
The former Chelsea and Inter Milan boss refused to be drawn on whether Casillas or Adan would start between the posts at the Bernabeu.
"They don't even know who is going to play in goal yet," he said.
"All I can say is, whoever plays against Real Sociedad will not play on Wednesday (in the King's Cup)."
Meanwhile, Argentina striker Gonzalo Higuain has returned to the Real squad for Sunday's game after around two months out with a hamstring injury.
Higuain is their second-highest scorer in La Liga with seven goals and will be a boost for Mourinho as he battles to get his side back on an even keel after a poor end to 2012, when they lost two and drew one of their last three games.
They were surprise 2-1 losers at promoted Celta Vigo in a Cup last-16 first leg and host the return on Wednesday.
In La Liga, they have already dropped more points this season than during the whole of their title-winning 2011-12 campaign, losing four games and drawing three, while leaders Barcelona have yet to lose in 17 matches.
Real, who are third, trail second-place Atletico Madrid by seven points and Barca by 16.
(For more sports news and updates, follow Ahram Online Sports on Twitter at @AO Sports and on Facebook at AhramOnlineSports)