Match ends, West Ham United 2, Manchester United 0.
West Ham manager Manuel Pellegrini said his side are becoming "more consistent and more solid" after continuing their fine start to the campaign with victory over Manchester United.
Goals from Andriy Yarmolenko and Aaron Cresswell earned the Hammers their second successive home win against the Red Devils, who lost striker Marcus Rashford with a groin injury in the second half.
The result lifts West Ham up to fifth in the table - 12 places and seven points better off than at the same stage last season.
"We have more quality, but we're also more consistent and more solid," said Pellegrini.
"It's good to win, especially against Manchester United here at home and to keep a clean sheet. We tried to win from the beginning.
"We didn't allow them to create chances and Andriy's goal was the key factor to open the game. It was a good goal."
Yarmolenko opened the scoring on the stroke of half-time, sending a low finish past David de Gea following patient build-up play involving Mark Noble and Felipe Anderson.
Cresswell sealed all three points for the hosts in the second half with a superb free-kick into the top right-hand corner.
Chances were at a premium in a cagey first half at London Stadium, with Noble's deflected effort from Pablo Fornals' free-kick the closest either team came to a breakthrough before Yarmolenko's strike.
"Maybe we played too fast in the first 45 minutes, which is why we lost so many balls," added the former Manchester City boss. "But I told the players at half-time that if we defend the way we did in the first half, we were not going to concede too many opportunities [in the second half]."
Juan Mata should have levelled for the visitors two minutes after the restart but failed to hit the target after connecting well with Andreas Pereira's low cross.
Rashford injury compounds Red Devils' woes
After making nine changes for the midweek Europa League victory over Astana, Solskjaer fielded the same team that beat Leicester at Old Trafford in their last league game.
Nemanja Matic and Rashford were the sole survivors from Thursday's win, with teenage striker Mason Greenwood - United's match-winner against the Kazakh side - unavailable due to tonsillitis.
Rashford, who had gone five matches without a goal in open play before today, looked short on confidence throughout, failing to register a single shot before going off injured just after the hour mark.
The injury capped a deeply frustrating afternoon for Solskjaer, whose side looked lacklustre, lethargic and short of ideas in the final third.
Matic's long-range drive, which was easily held by Lukas Fabianski, was the closest they came to a goal in a forgettable first half.
The visitors improved marginally in the second and should have restored parity when Mata got on the end of Pereira's delivery, but the veteran midfielder somehow managed to steer the ball wide from point-blank range.
Harry Maguire also went close to bringing the visitors level before West Ham's second goal, firing straight at Fabianski after the Hammers had failed to clear a corner.
The defeat extends Manchester United's poor away form - their last league victory on their travels came at Crystal Palace on 27 February.
Yarmolenko stars for Hammers
Pellegrini's charges were bottom of the table after four matches last season, but Sunday's result lifts them three points above Solskjaer's side in the standings - a mark of their progress under the Chilean's stewardship.
The Hammers have now kept four successive clean sheets in all competitions, while summer signing Sebastien Haller and fit-again Yarmolenko have shown considerable promise up front in recent weeks.
Yarmolenko was a constant menace, breaking the deadlock with a composed finish and registering more shots than anyone else on the pitch.
The Ukrainian nearly set up West Ham's second midway through the second half, but Felipe Anderson - who endured a disappointing afternoon - fired straight at De Gea from a tight angle.
The game remained on a knife edge until the 84th minute, when Cresswell's sublime free-kick - his first goal since April 2018 - sealed the points for the home side.
The result moves Pellegrini's team level on points with third-placed Leicester.
Man of the match - Andriy Yarmolenko (West Ham)
Man Utd vulnerable on their travels - the stats
- After winning each of his first nine away games in all competitions as Man Utd boss, Ole Gunnar Solskjaer has failed to win in the following nine (D3 L6).
- West Ham have won three of their last five Premier League home games against Man Utd (D1 L1), as many as they had in their previous 23 home games against them in the top flight (W3 D11 L9).
- Manuel Pellegrini is the first manager to win a Premier League game against four different Manchester United managers (David Moyes, Louis van Gaal, Jose Mourinho and Solskjaer).
- Man Utd have failed to keep a clean sheet in any of their last 11 away matches in all competitions, their worst run since conceding in 14 consecutive matches between April and December 2002.
- Man Utd have lost seven of their last 15 Premier League games (W4 D4 L7), as many defeats as they had recorded in their previous 40 matches in the competition (W25 D8 L7).
- Since the start of the 2011-12 season, David de Gea has conceded more Premier League goals from direct free-kicks than any other goalkeeper (12).
- Felipe Anderson has been directly involved in 15 Premier League goals for West Ham since the beginning of last season (9 goals, 6 assists) - more than any other Hammer in that period.
'We're just all very flat' - what the managers said
West Ham manager Manuel Pellegrini, speaking to BBC Sport: "The international break was important for us because we had all the defenders and worked a lot to be more concentrated, to make better tackles and better movements.
"I think we must try to beat Bournemouth 1-0 in our next league game and then see what happens. We're not thinking about the future."
Man Utd manager Ole Gunnar Solskjaer, speaking to BBC Sport: "I'm very disappointed. You're always disappointed when you lose games. This was a game that we could have won. Going home tonight we'll go through the game again. At the moment we're just all very flat.
"Key moments went against us. They had some great finishes. We just didn't have the quality when we had those big moments. In the Premier League if you don't take them you won't get any points. It tipped in their favour and we just have to accept that.
"I'm disappointed we didn't win today, but apart from that I'm being very positive. I've said many times that there will be highs and lows. We've had some highs along the way, today we just have to accept we got no points and look forward to next week."
West Ham travel to Oxford United in the third round of the Carabao Cup on Wednesday, 25 September (19:45 BST) before visiting Bournemouth in the Premier League on Saturday, 28 September (15:00 BST).
After hosting Rochdale in the Carabao Cup on Wednesday (20:00 BST), Man Utd welcome Arsenal to Old Trafford in their next league game on Monday 30 September (20:00 BST).
- 24FredericksSubstituted forZabaletaat 80'minutes
- 21OgbonnaBooked at 14mins
- 7YarmolenkoSubstituted forSnodgrassat 89'minutes
- 16NobleBooked at 74mins
- 8Felipe AndersonSubstituted forWilshereat 70'minutes
- 15C Sánchez
- 1de Gea
- 18YoungBooked at 83mins
- 31MaticSubstituted forFredat 71'minutes
- 8MataBooked at 48minsSubstituted forGomesat 81'minutes
- 10RashfordSubstituted forLingardat 61'minutes
- Anthony Taylor
- Shots on Target
Second Half ends, West Ham United 2, Manchester United 0.
Substitution, West Ham United. Robert Snodgrass replaces Andriy Yarmolenko.
Scott McTominay (Manchester United) wins a free kick in the defensive half.
Foul by Andriy Yarmolenko (West Ham United).
Attempt missed. Daniel James (Manchester United) left footed shot from the left side of the box is close, but misses to the left. Assisted by Fred following a fast break.
Foul by Daniel James (Manchester United).
Pablo Zabaleta (West Ham United) wins a free kick on the right wing.
Attempt blocked. Andreas Pereira (Manchester United) right footed shot from outside the box is blocked. Assisted by Fred.
Goal! West Ham United 2, Manchester United 0. Aaron Cresswell (West Ham United) from a free kick with a left footed shot to the top right corner.
Ashley Young (Manchester United) is shown the yellow card for a bad foul.
Foul by Ashley Young (Manchester United).
Mark Noble (West Ham United) wins a free kick in the attacking half.
Substitution, Manchester United. Angel Gomes replaces Juan Mata.
Substitution, West Ham United. Pablo Zabaleta replaces Ryan Fredericks because of an injury.
Corner, Manchester United. Conceded by Andriy Yarmolenko.
Attempt blocked. Jesse Lingard (Manchester United) right footed shot from the centre of the box is blocked.
Offside, West Ham United. Angelo Ogbonna tries a through ball, but Sébastien Haller is caught offside.
Attempt missed. Aaron Cresswell (West Ham United) left footed shot from outside the box is too high from a direct free kick.
Jack Wilshere (West Ham United) wins a free kick in the attacking half.