According to Caio Carrieri of UOL (h/t Lucas Sposito of Sport Witness), some senior members of the team have been left shocked by the marginalisation of the Brazil international, who was the club's major summer signing from Shakhtar Donetsk.
Carrieri wrote when Mourinho was trying to convince Fred to move to the club he advised him he would be part of a "dream midfield" alongside Paul Pogba and Nemanja Matic.
However, so far this season the 25-year-old has made just 10 appearances for his new team, including just six starts in the Premier League.
Carrieri reported "even the athletes with more authority in the squad and with more time at the club" have been left confused by the way in which the £47 million midfielder has been handled.
Speaking about the player on Friday, Mourinho explained some of the reasoning behind his decision to keep the midfielder out of the side, per Simon Stone of BBC Sport:
There's no doubt that United need to improve in defence, as they've struggled throughout the Premier League campaign to keep opposing teams out.
Mourinho has started with numerous different systems and personnel in at the back, but has still watched his side leak goals:
While it's easy to look at the defenders and blame them for these issues, if United were more resourceful in other areas of the field that may alleviate some of the pressure.
Ahead of the defence, the United midfield has lacked balance, with Pogba's form inconsistent and Matic a long way short of his usual standards. With that in mind, it's curious a footballer in whom the Red Devils invested so much hasn't been shown more faith.
After all, Fred is the kind of player who would make a big difference at his best. Other facets of Mourinho's side appear stale at the moment, whereas as during his time at Shakhtar the Brazilian was vibrant, positive on the ball and diligent in his defensive work.
Even so, Ciaran Kelly of the Manchester Evening News noted this is a ploy Mourinho has used before:
With the team playing uninspired and fractured football, it's not a surprise the Red Devils fanbase want to see something fresh from the team. A run of games for Fred would be welcome as a result.
Yet Mourinho is renowned for being pragmatic and occasionally stubborn in his approach, meaning the Brazilian may have to wait a while yet before he gets a prolonged chance to make his mark.