Casillas suffered a fracture in his hand in January that prompted the Spanish club to sign Lopez from Sevilla.
"We must be honest," ESPN quotes Mourinho as saying. "The way Diego is playing, it will be very hard to get him out of the net.
"I cannot say for certain that he will be there till the end of the season, but he is playing very well. For now, it will be difficult for Casillas to be on the bench against Galatasaray, but he will surely be there for Levante."
Madrid will host Galatasaray in the first leg of their Champions League quarter-final tie on Wednesday.