Crystal Palace claimed their first victory of the season with a 1-0 win over Tottenham.
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Liverpool can go back to the top of the table with victory away at Arsenal in the biggest clash of the Premier League weekend later on Sunday.
Ten Hag was left to rue a familiar lack of ruthlessness as United's fourth league defeat in nine games will spark further speculation on how long he will be given to turn the Red Devils' fortunes around.
The visitors should have been out of sight by half-time at the London Stadium.
Alejandro Garnacho hit the woodwork inside two minutes and Edson Alvarez also headed off his own crossbar.
But it was Diogo Dalot who missed the biggest chance when the Portuguese international somehow fired over with the goal gaping after rounding Lukasz Fabianski.
Under-fire West Ham boss Julen Lopetegui showed his disgust with the Hammers' first-half display by making three substitutions at the break.
Crysencio Summerville was one of those introduced and he slid in to open the scoring against the run of play on 74 minutes.
Casemiro brought United level nine minutes from time with a header from point-blank range.
But there was late drama when VAR intervened for a foul on Danny Ings by Matthijs de Ligt.
Jarrod Bowen smashed in the resulting spot-kick to take West Ham above United, who sink to 14th in the table.
Chelsea had been beaten in the Premier League for the first time since Enzo Maresca's first match in charge at Liverpool last weekend despite an impressive performance.
The Blues got back to winning ways with Palmer at the heart of their best work.
The England international had a goal marginally ruled out for offside inside the first five minutes.
Palmer was the creator for the opening goal, despite not being credited with the assist, as his sumptuous ball over the top freed Pedro Neto, who picked out Nicolas Jackson to slot in his sixth goal of the season.
Newcastle are now winless in five league games as the pressure mounts on Eddie Howe ahead of a rematch against Chelsea in the League Cup on Wednesday.
The Magpies did get back on level terms before half-time as Alexander Isak tapped in Lewis Hall's cross at the end of a well-worked move.
Palmer took just two minutes into the second period to make the decisive impact with a driving run forward and powerful finish that beat Nick Pope at his near post.
Newcastle should have left London with a point, though, as Isak botched a huge chance to level 15 minutes from time after rounding Robert Sanchez.
Palace began the day in the relegation zone but were good value for the three points at Selhurst Park as Tottenham's struggles on the road continue.
Eberechi Eze's deft flick teed up Jean-Philippe Mateta to fire home the only goal.
Eze had a second ruled out for offside in the second half, but Tottenham, without injured captain Son Heung-min, were toothless in attack.
Spurs have won just once in five away league games this season as they slip to eighth, four points off the top four.
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