Fred started the 2018/19 season brightly with a promising performance on his debut against Leicester back in August, but Mourinho worries he isn't the right player at a time when United are leaking an average of 1.8 goals per Premier League game.
There is also an element of adapting to an alien country and new type of football after spending the previous five years with Ukrainian giants Shakhtar Donetsk.
"Step by step [Fred is adapting]," Mourinho said as he faced the media at his weekly press conference, via ManUtd.com.
"You also have other players at other clubs that needed their time and you have some of them that play even less than what Fred is playing.
"I think Fred [will play more], when the team is defensively stronger and doesn't need in midfield people that are more worried about giving some balance to the team than being involved in creation and attacking dynamics.
"The day we are stronger defensively, I think the horizons for Fred change completely."
Fred was dropped following United's 3-0 home defeat at the hands of Tottenham at the end of August and has only featured in six games in all competitions since. He hasn't got on the pitch in the Premier League at all since being hooked after 56 minutes against Bournemouth last month.
United face an even leakier defence than their own in the shape of Fulham on Saturday and are looking to break an ongoing win-less streak that has so far included game with Manchester City, Crystal Palace, Southampton and most recently Arsenal.
"I want to be positive and say we are going to win, but we have to give credit to [Fulham], it will not be 11 against zero. It will be 11 against 11," Mourinho commented.
United staff will make late decisions over the fitness of Anthony Martial, Phil Jones, Eric Bailly and Chris Smalling after suffering knocks. Luke Shaw is more likely to return after missing the Arsenal draw, while Ashley Young is available after suspension.