Fred, who joined United from Shakhtar Donetsk in the close season for a fee reported by British media to be 52 million pounds ($66.29 million), has started only six of their 15 Premier League matches.
He was left out of the squad for Wednesday's 2-2 draw with Arsenal and a lack of regular playing time has seen the 25-year-old lose his spot in the Brazil squad.
Mourinho, however, said that Fred would get his opportunities in time.
"Step by step, you have also other players in other clubs that needed their time and you have some of them that played even less than what Fred is playing," Mourinho told a news conference ahead of Saturday's clash with Fulham.
"I think Fred, when the team is defensively stronger and doesn't need in midfield people more worried in giving balance than being involved in creation... the day we are stronger defensively, the horizons for Fred change completely."
United have conceded 25 goals this season, one more than they have scored, and sit eighth in the league with 23 points --eight points adrift of the top four.
Mourinho said United, who have drawn their last three league matches, could offer no excuses if they lose to Claudio Ranieri's Fulham, who are bottom of the league with nine points.
"I know only two days (to prepare) is not easy. Fulham in the same situation as us but probably the way they are coming to play or try to play will not demand as much from them as us.
"But no excuses, I sat here a few weeks ago speaking about the December month ... good fights, good spirit, but one victory against Young Boys and three draws and we need points, but Fulham is the same."
Mourinho will assess the fitness of forward Anthony Martial and the defensive trio of Phil Jones, Luke Shaw and Chris Smalling before the match at Old Trafford.
($1 = 0.7844 pounds) (Reporting by Shrivathsa Sridhar in Bengaluru Editing by Toby Davis)
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