The Red Devils completed a surprise swoop to sign the experienced goalkeeper from relegated Stoke City earlier this month.
Grant’s move came as something as a shock given the 35-year-old is in the twilight of his professional career.
The former England Under-21 international has played for Derby County, Burnley, Oldham Athletic, Sheffield Wednesday, Stoke City before his move to Manchester United.
Pereira has spent the last couple of seasons as third choice goalkeeper behind current number one David De Gea and Argentina international Sergio Romero.
Speaking to the media for the first time this summer ahead of Manchester United’s pre-season friendly against Club America, Mourinho shed light on the decision to sign Grant.
“We did Lee Grant because we have a fantastic young goalkeeper who needs to play football,” Mourinho told his media conference on Wednesday.
“Joel is a Manchester United goalkeeper but two years without football he has to play. And to protect that position we got Lee, experienced in the Championship, with Stoke City in the Premier League, so he is ready to fulfil that position.
“Joel is here with us, [it’s] is good for him to play but then he will go on loan.”
Manchester United have also signed FC Porto full-back Diogo Dalot in a £17m deal to bolster his options at full-back ahead of the departure of Matteo Darmian.
The Red Devils spent £52m on Brazil international Fred to see off competition from Premier League champions Manchester City in the Shakhtar Donetsk midfielder.
The 20-time English champions lost 1-0 to Chelsea FC in the FA Cup final at Wembley.
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