Pogba would consider either a move to Barcelona or a return to Juventus if given the option, the sources added, as the midfielder is unhappy with some of his manager's comments made during the club's tour of the United States.
"Pogba came back to United with this great expectation, that he had to show the world how good he is straight away", Brown told The Express.
Mourinho's decision to keep Pogba at Old Trafford has been reported by Miguel Delaney of the Irish Independent.
According to Sport, the representative has a plan to present the Red Devils' board with a £100 million (R1.73 billion) offer for the 25-year-old.
United are said to have three reasons to have rejected Barcelona's supposed bid for Pogba.
"This season, especially after the World Cup that he's had, he's going to start expressing himself a lot more".
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Pogba would be the final piece of the jigsaw as Barcelona are unrelenting in his pursuit, but it's on the player's part whether to agree to a pay cut or stay put with a higher wage bill. I think the World Cup is the ideal habitat for a player like him to give the best.
Pogba's midfield partner Nemanja Matic is also sidelined following abdominal surgery in America.
'The players are in a closed camp, isolated from the external world, from commercial compromises, and every possible influence'.
Both players had already been linked with moves to England already, with Everton chasing Mina and West Ham reportedly interested in Gomes.
He is under contract with Manchester United until 2021.
Mina, Leicester's Harry Maguire and Tottenham's Toby Alderweireld have been the alternative targets as Old Trafford executive vice-chairman Ed Woodward looks to deliver the new centre-back that Mourinho wants.