The Portuguese acknowledged the potential selection headache he could face selecting his first-choice custodian, as captain Iker Casillas nears a return to action
The winter arrival, signed in response to Casillas' bone fracture in January, has surpassed expectations in the skipper's absence, putting in a series of assured performances for los Blancos.
And Mourinho's acknowledgement of such has placed the Madrid captain's squad status in doubt, when he told reporters: "We must be honest. 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 stay 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."
The Spanish champions drew 1-1 against Zaragoza on Saturday after Cristiano Ronaldo cancelled out Rodri's early opener, prompting Mourinho to lament his team's scoring touch deserting them this season.
"It is true that all our players except Ronaldo have scored less goals this season, and therefore the team suffers," the 50-year-old continued.
"But I believe they will come, if not this season, then the next. It is not a matter of winning the league now, but to secure second place.
"Our goals remain the same; to progress to the Copa del Rey final and beyond the quarter-finals of the Champions League."
Madrid host Galatasaray in Europe's premier club competition on Wednesday.