Fred, who joined United from Shakhtar Donetsk for £52million, has started just three of his new club’s last 14 matches.
The 25-year-old was left out of the matchday squad entirely for Wednesday’s draw with Arsenal, while his last domestic appearance came against Bournemouth on 3 November.
Fred’s lack of playing time has puzzled United supporters, but Mourinho explained that the Brazil international’s “horizons” will change once his side tighten up at the back.
The United manager said that Fred, who was also linked with Manchester City was adapting to life in English football “step-by-step”.
“You have also other players in other clubs that needed time, you have some that play even less than what Fred is playing.
“I think Fred, when the team is defensively stronger, and does not need him in midfield - people that are more worried in giving some balance to the team than being involved in creation and attacking dynamics - the day we are stronger defensively, the horizons for Fred change completely.”
United will hope to end a four-game winless run in the Premier League when they welcome bottom-of-the-table Fulham to Old Trafford on Saturday.
Fulham have conceded more goals than any other top-flight side, though the Cottagers have seen signs of improvement since Claudio Ranieri's appointment and Mourinho is expecting a difficult afternoon against the basement club.
“I want to be positive and think we are going to win but if we don’t give credit to the people that deserve credit,” Mourinho said.
“The game is not going to be 11 against 0, it’s going to be 11 against 11, good players in other side.
“I repeat the position, the points sometimes don’t say much about the real potential of the team, one of the two most important managers in the history of the Premier League, so they have the potential to come here tomorrow and to give us a very difficult match.”
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