The Portuguese boss endured a frustrating transfer window as chief executive Ed Woodward failed to bring in his primary transfer target of a central defender, signing only Fred, Diogo Dalot and Lee Grant during the summer.
Rumours of a fallout between Mourinho and Woodward have been rife since the transfer window closed, and The Mirror reports that the former Chelsea boss is still seething at the club's lack of ambition in the market.
Mourinho reportedly believes that his chances of closing the gap on champions Manchester City have been undermined by Woodward's refusal to spend the money needed to bring in a new centre-back.
The 55-year-old is understood to have told friends that he would have left any other club in protest had he been treated the same, but only stayed because of Manchester United's stature.
United's defensive frailties were exposed in just their second game of the season, when they were beaten 3-2 by Brighton & Hove Albion last weekend.