City conceded a goal inside two minutes on Sunday and were overwhelmed in the second half in a humbling 5-1 loss at Arsenal, which trimmed the Gunners' gap to leaders Liverpool to six points.
Earlier, United had lost again at home, 2-0 to Crystal Palace, and now have seven defeats in 13 league games at Old Trafford this season — something not seen since the 1890s.
Erling Haaland and Pep Guardiola contemplate a humbling defeat at Arsenal. (AP PHOTO)
But while City boss Pep Guardiola and his United counterpart Ruben Amorim licked their wounds, at least another under-pressure boss Ange Postecoglou was enjoying some blessed relief with his Spurs side winning 2-0 at Brentford for their first EPL win in 49 days
The chances of City winning a fifth straight title are surely over — the champions now 15 points behind Liverpool with 14 matches remaining — and they must have felt humiliated by the end of the game at the Emirates Stadium that again demonstrated the new-found hostility between Arsenal and City.
All is evidently still not right with Guardiola's team, who let in a sloppy opener when Manuel Akanji was dispossessed near his own area and Martin Odegaard scored.
Erling Haaland was goaded after that goal by Gabriel in a hangover from the 2-2 draw between the teams in September, but responded by equalising in the 55th.
Arsenal then went back in front in the 57th through Thomas Partey's deflected shot and there were further goals from the Myles Lewis-Skelly, Kai Havertz and Ethan Nwaneri.
Lewis-Skelly marked his goal by sitting down, crossing his legs and delivering a "Zen" celebration — one often used by Haaland.
Arsenal are now alone in second, three points above Nottingham Forest and nine above City.
On the day, Marcus Rashford edged closer to a departure from Old Trafford, United lacked a cutting edge up front.
And there was more bad news for them with a tearful Lisandro Martinez carried off on a stretcher with a left knee injury.
Jean-Philippe Mateta scored both goals in the second half for Palace to end United's three-match winning run in all competitions – a streak coming after manager Ruben Amorim had called his team "the worst team, maybe, in the history of Manchester United."
The Amorim era looks set to be without Rashford, with the forward reportedly undergoing a medical examination at Aston Villa ahead of sealing a loan move with a view to a permanent transfer.
United have lost seven of their first 13 home games in the Premier League this season which hasn't happened since 1893-94.
United dropped to 13th in the standings, just one place and two points ahead of Tottenham, whose win ended a four-match losing run in the league.
Tottenham's Pape Sarr, arms aloft, celebrates Spurs' clincher at Brentford. (AP PHOTO)
Vitaly Janelt scored an own-goal at a corner to put Tottenham ahead at half-time and Pape Sarr added the second goal in the 87th.
"We needed a big effort again today, a physical effort. We put that in," said Postecoglou, who rested defender Micky van de Ven after the defender returned from injury in the midweek win over Elfsborg in the Europa League.
"Outstanding effort by the players."
The result was a huge boost for Tottenham ahead of their biggest match of the season, the second leg of the League Cup semi-final against Liverpool on Thursday, with Spurs leading 1-0 from the first leg.
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