The Portuguese coach reportedly told Fred that he would be part of a "dream" midfield trio consisting of Paul Pogba and Nemanja Matic, which triggered his move to Old Trafford.
United signed Fred for £52m from Shakhtar Donetsk in the summer but the Brazilian has struggled to secure regular playing time as he is said to be adjusting to new conditions in England.
Fred has started just six Premier League and two Champions League games for the Red Devils in what has been a testing time for the player since making the move to United.
Metro quote UOL in reporting that many players in the squad have been left confused as to why Fred has been overlooked by Mourinho so many times this season.
The report then went on to claim that during negotiations with Fred, Mourinho personally told the Brazilian that he will play an integral role for United with Pogba and Matic. But the Portuguese coach has started the trio just four times this season.
Mourinho was recently asked about the 25-year-old midfielder not getting enough playing time to which the coach replied, "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."
He concluded by stating, "I think [with] Fred, when the team is defensively stronger and doesn’t 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."
Fred will be hoping he will get his chances, especially with the FA Cup coming in January, which could give Mourinho an option to start using the Brazilian more often.