The Reds signed the Brazil international in a £52m deal from Ukrainian side Shakhtar Donetsk in the summer transfer window.
Fred was initially linked with a move to their bitter derby rivals and Premier League champions Manchester City.
The 25-year-old opted to link up with Manchester United following five seasons in the Ukrainian top flight.
Fred will be tasked with helping the Red Devils to fill the void left by former England international Michael Carrick.
Mourinho shed light on Manchester United’s decision to sign the South American midfielder in the current transfer window.
“Fred, you know we did it quite a long time ago, we started the process a long time ago,” Mourinho told his press conference on Wednesday.
“We lost Michael Carrick, we think Fred is the kind of player we need to complement the qualities of our midfield players.”
Manchester United finished in second place in the Premier League table last term after the Old Trafford outfit ended 19 points adrift of champions Manchester City.
The Red Devils lost 1-0 to Chelsea FC in the FA Cup final at Wembley after Eden Hazard netted the winner from the penalty spot.
Manchester United have also signed Portuguese defender Diogo Dalot in a £17m deal from FC Porto to bolster Mourinho’s options in defence ahead of the new season.
Mourinho has won the League Cup and the Europa League since replacing Louis van Gaal at Old Trafford.
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