Fred arrived from Shakhtar Donetsk in the clubs biggest signing of the summer, but has made just 11 appearances in total and has not featured in Uniteds last four Premier League games.
The Brazilian did not even make the squad for the visit from Arsenal, and United fans are scratching their heads as to why Fred is continually overlooked.
United welcome Fulham to Old Trafford on Saturday and speaking ahead of the game, Mourinho explained it was impossible to play Fred at the moment.
He said: You know, step-by-step. Step-by-step. You also have other players in other clubs that they needed their time and you have some of them that play even less than what Fred is playing.
I think Fred, when the team is defensively stronger, doesnt need in midfield people that is more worried in 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.
Mourinho has often hit out at his players for their lack of effort and defensive awareness this season, with his side currently performing well below the expectations of United fans.
United fans will expect three points against relegation-battlers Fulham, and Mourinho confirmed he is still targeting no defeats in the month of December.
Fulham are in the same situation as us [needing points] but probably the way they are coming to play, or try to play, will not demand as much from them as us, said Mourinho.
But no excuses. I sat here a few weeks ago speaking about December and no defeats, thats a reality.
Good fights, good spirit, but just one victory, against Young Boys, and three draws, and we need points.
I want to be positive and think we are going to win, but if we dont, give credit to the people that deserve credit.
The game is not going to be 11 against zero, its going to be 11 against 11.
They have one of the two most important managers in the history of the Premier League [in Ranieri], so they have the potential to come here tomorrow and to give us a very difficult match.