The England international has struggled to hold down a regular starting spot in Mouirinho’s side over the 12 months or so.
The 20-year-old has been in and out of Mourinho’s starting line up to raise questions about his long-term future at the 20-time English champions.
Rashford was linked with a potential move in the summer transfer window but a deal didn’t materialiise and Martial remained at Old Trafford.
The England striker has started one game and made two substitute appearances in the Premier League so far this term.
Rashford has been linked with a potential exit at Old Trafford ahead of the January transfer window.
Sunday Times reporter Castles explained why Mourinho wants to keep Rashford at the 20-time English champions.
“Mourinho thinks Rashford can turn into a top player mainly because of his mental attitude and willingness to learn on top of the technical attributes that he’s got,” Castles told the Transfer Window podcast.
“Rashford still has to prove he can develop into that type of player.”
Rashford scored eight goals in 18 games under Louis van Gaal at Old Trafford in 2015-16.
The England star has won the League Cup and the Europa League since Mourinho replaced Van Gaal.
The Red Devils star was handed the number 10 shirt at the start of the new Premier League season.
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