According to Matt Lawton of the Daily Mail, his frequent trips back to London are causing concern at Old Trafford, and club staff "regard him as an increasingly detached figure," while one source reportedly said on Wednesday: "Jose seems to be having third-season syndrome a year early with us."
"Third-season syndrome" references the phenomenon in Mourinho's career that has seen him win domestic league titles in his second season at multiple clubs before departing in the following campaign.
Never was it more apparent than in his second spell with Chelsea. He was reappointed in June 2013 ahead of a campaign in which the Blues finished a respectable third in the Premier League.
He then masterminded a dominant Premier League title win in 2014-15 before being sacked in December 2015 with the defending champions just one point above the relegation zone.
In Mourinho's first season with United in 2016-17, the Portuguese led the Red Devils to EFL Cup and UEFA Europa League glory, the latter earning the Manchester giants a place back in the UEFA Champions League.
Seven matches into the current Premier League campaign, Mourinho's second with United, it looked as though the pattern could be repeating itself as the Red Devils were level with Manchester City at the top of the table.
Since then, though, United have fallen way off the pace as Pep Guardiola's superlative Sky Blues have gone unbeaten.
Mourinho's side are still in second place, but they are 15 points behind City, and the United boss has recently shown signs of feeling the pressure.
The 54-year-old's latest indiscretion has been to take a swipe at Old Trafford legend Paul Scholes after his criticism of Paul Pogba, per BBC Sport.
As noted by Lawton, Mourinho's decision to live at a hotel since he took over at United—his home is in London—is in stark contrast to his predecessor, Louis van Gaal, who settled in Cheshire with his wife.
United are better off under Mourinho than they were with Van Gaal. They have 10 more points now after 22 matches of the season than they did at the same stage in the Dutchman's final term in charge of the club.
However, as City run away with the title, Mourinho's future at United is looking increasingly uncertain.