The Spanish captain has recently returned to full training after breaking a bone in his hand back in January.
However, after another fine performance by Diego Lopez in Madrid's 1-1 draw with Real Zaragoza on Saturday, Mourinho said it will be difficult for Casillas to regain his place.
“On Wednesday Casillas won’t be a substitute. Against Levante he will be in the squad,” he told a press conference.
“I can never say that a player will start for the rest of the season, but I can say that once more Diego Lopez has played very well and helped the team obtain a result.”