He is yet to feature for United in pre-season after World Cup duty with the Brazilian national team.
The 25-year-old will be a welcome addition to United’s midfield this term following the departure of Michael Carrick, who retired at the end of last season to take up a coaching role at the club.
But Mourinho believes Fred will bring creativity and flair to the centre of the park – and something different to the long-serving midfielder.
"Nemanja Matic, Marouane Fellaini, Paul Pogba and even Ander Herrera are physical and strong players, he told reporters.
"We think having a player like Fred in the squad – who is more technical, more quality of passing in short spaces – he can maybe connect [with] the attacking players.
"He could be an important player for us, but let’s wait. Michael Carrick is Michael Carrick and there is no successor."
United continue their pre-season campaign on Wednesday when they host Serie A side Milan in Pasadena.
The Red Devils have played out two draws against Club America and San Jose Earthquakes so far.