Erik ten Hag's side produced a wretched display that was punished by Morgan Gibbs-White's late winner at the City Ground.
Nicolas Dominguez had put Forest ahead in the second half before Marcus Rashford equalised with just his third goal of a woeful campaign.
United's fourth loss in their last six games in all competitions left them languishing in seventh place in the Premier League, nine points adrift of the top four.
United's stirring fightback from two goals down to beat Aston Villa 3-2 on Tuesday had been heralded as the start of a new era just days after Jim Ratcliffe's investment in the troubled club.
British billionaire Ratcliffe purchased a 25 percent stake in United for around £1.03 billion ($1.3 billion) on Christmas Eve.
Ratcliffe's INEOS Group will take control of United's football operations from unpopular owners the Glazer family.
But despite Ten Hag's claim on Friday that Ratcliffe wants to work with him, this was the latest evidence that the new regime might consider dismissing the beleaguered Dutchman.
INEOS's head of sport Dave Brailsford was watching alongside the club's former boss Alex Ferguson at the City Ground as United were beaten by Forest for the first time since 1994.
Ten Hag will have to oversee a significant improvement in United's results and performances to avoid the axe.
Forest moved five points clear of the relegation zone after Nuno Espirito Santo masterminded his second successive high-profile victory.
Nuno's first win as Forest boss after replacing the sacked Steve Cooper came against Newcastle, but this was an even bigger scalp.
- Turgid United -
After their slow start against Villa, Ten Hag's men once again laboured to find any rhythm as passes went astray with alarming regularity.
Forest had conceded more goals than any other Premier League team in 2023, but it took 32 minutes for United to muster their first shot.
Even then, Aaron Wan-Bissaka's deflected effort trickled tamely to Forest keeper Matt Turner.
Ten Hag's furrowed brow as he stood glumly on the touchline painted a vivid picture of United's struggles.
Elanga was allowed to leave United in the close-season, while the underachieving Antony, hand-picked by Ten Hag from his old club Ajax, remains in the team.
However, it was Elanga, playing with a point to prove, who looked the more dynamic force and it was no surprise when Antony was hauled off after 53 minutes, leaving the Brazilian without a goal or assist in 21 appearances.
United striker Rasmus Hojlund, who scored his first Premier League goal to seal the Villa victory, was sidelined due to illness.
Rashford, playing as United's central striker in Hojlund's absence, might have ended up on the scoresheet, but he hardly looked convincing in the role.
Diogo Dalot fired against the post from 25 yards as United finally pieced together an incisive attack.
But United's turgid display got what it deserved in the 64th minute when Gonzalo Montiel's pass picked out Dominguez and he slotted a cool finish past Andre Onana from 12 yards.
As Forest fans serenaded Ten Hag with chants of "you're getting sacked in the morning", United's Alejandro Garnacho looped a half-volley onto the roof of the net from Bruno Fernandes' corner.
Turner gifted United an equaliser with 12 minutes left when his woeful pass was intercepted by Garnacho, who teed up Rashford to fire past the red-faced keeper from 10 yards.
But even that belated Christmas present wasn't enough to save United, who combusted again in the 82nd minute.
Undone by sloppy marking as Elanga provided the assist, United stood statuesque as Gibbs-White curled a superb strike past Onana from the edge of the area.
Making amends for his earlier mistake, Turner saved from Fernandes in stoppage-time to leave United in turmoil.