The 20-time English champions entered Saturday's clash with Nottingham Forest off the back of a 3-2 victory over Aston Villa at Old Trafford on Boxing Day.
However, the Red Devils were unable to build on the positive result, suffering a 2-1 defeat at Forest.
Marcus Rashford levelled the scores in the 78th minute after Nicolas Dominguez had made the breakthrough in the 64th minute, but Morgan Gibbs-White won it for the hosts in the 82nd minute.
The defeat was Man United's 14th in all competitions this season, which is their most before the turn of the year since the 1930-31 campaign.
There were more unwelcome statistics to come out of the loss at Forest, with Man United suffering nine defeats in their first 20 league games of a campaign for the first time since the 1989-90 campaign.
What's more, the 20-time English champions have suffered 21 defeats in all competitions in 2023 - only on three occasions have they lost more times in a calendar year, which came in 1930 (28), 1972 (25) and 1921 (24).
The result has left Man United down in seventh position in the Premier League table on 31 points, boasting a record of 10 wins, one draw and nine losses from 20 matches.
Erik ten Hag's side are nine points behind fourth-placed Arsenal, who have a game in hand, while they also finished bottom of their Champions League group, suffering four losses.
The team's other defeat this season came at the start of November against Newcastle United in the EFL Cup.
Man United will not be back in Premier League action until January 14 at home to Tottenham Hotspur, but they will visit Wigan Athletic in the third round of the FA Cup on January 8.
The Red Devils could then be in FA Cup fourth-round action at the end of next month before opening February with a trip to Wolverhampton Wanderers in the Premier League.
Ten Hag is expecting to recover a number of important players from injury in the early stages of 2024, with Harry Maguire, Lisandro Martinez, Casemiro and Mason Mount set to be among those to return.