The Red Devils' seven-game unbeaten run - which had stood since the start of the year - and five-match winning streak crashed and burned against the Cottagers, whom they faced without in-form striker Rasmus Hojlund due to a muscle injury.
After sixty-five minutes of goalless action at Old Trafford, the visitors drew first blood through Calvin Bassey, but Harry Maguire had seemingly stolen a point for the Red Devils with his 89th-minute effort.
However, seven minutes into second-half injury time, ex-Arsenal man Alex Iwobi left Erik ten Hag and co dumbfounded with the winning goal for Fulham, who condemned Man United to their 10th top-flight loss of the 2023-24 season.
Ten Hag's men had ignited a top-four charge thanks to their stellar start to 2024, but the sixth-placed Red Devils are now eight points behind Aston Villa in the final Champions League place, while Tottenham Hotspur are three points better off with a match in hand.
According to The Guardian, a portion of unnamed Man United players told coaching staff that they wanted a day off on Sunday, in spite of their disappointing defeat and the fact that analysis and warm-down sessions usually take place the day after a game.
Ten Hag was apparently not made aware of the request, which was rejected by "surprised" coaches, who instead reminded the players of their responsibilities and ordered them to report for training at Carrington as normal.
Man United broke several unwanted records during a dismal start to the 2023-24 season, which some players allegedly felt was due to tiredness from a brutal pre-season under Ten Hag.
The Dutchman made an immediate positive impact at Old Trafford with a third-placed Premier League finish and EFL Cup triumph in 2022-23, but his future has been a hot-button topic during the Red Devils' well-documented troubles this term.
INEOS founder Sir Jim Ratcliffe has now completed his 27.7% purchase of the club, which has been ratified by the Premier League and FA, and the 71-year-old's team will have full control over football operations as part of their internal makeover.
Ten Hag's contract at Old Trafford runs until the end of next season, and the former Ajax head coach is expected to sit down with Ratcliffe and discuss his future this summer, but he is not believed to be in danger of the sack just yet.
However, Ratcliffe supposedly does not feel that Ten Hag is the coach to take Man United forward in the long-term, and the minority shareholder is believed to be running the rule over potential replacements.
Furthermore, Ten Hag and Ratcliffe are at risk of coming to blows over their stance on transfers, as the former has demanded a large degree of control over incomings and outgoings, which he may not retain under the new regime.
Man United are back in action on Wednesday night, travelling to Nottingham Forest for the fifth round of the FA Cup before the weekend's blockbuster Manchester derby with Manchester City at the Etihad.