Jarrod Bowen's 90th minute penalty was enough to earn the hammers all three points in this battle of the Uniteds.
The club captain struck from 12 yards to secure a vital win for Julen Lopetegui's side, just as it seemed that both sides would have to settle for a point apiece.West Ham certainly rode their luck in the opening 45 minutes of this entertaining encounter, with the visitors hitting the woodwork twice through Garnacho and an Edson Alvarez header while Dallo missed an open goal, to the vast amusement of the home crowd.
Yet it was a different game after the break following a number of half time adjustments by Lopetegui and his team. West Ham, shaper in the tackle and entertaining a higher press offered the Mancunians far fewer opportunities while beginning to make inroads themselves.
And that more aggressive attitude finally paid dividends with 74 minutes on the clock when a low Bowen cross was turned on by Lucas Paqueta and slammed into the room of the net by substitute Crysencio Summerville, registering his first goal in a claret and blue shirt.
The former Leeds man, who enjoyed a productive afternoon duly raced around the perimeter of the pitch to celebrate with the suspended Mo Kudus, whose absence had offered Summerville an opportunity to feature.
West Ham's joy was to last for just seven minutes however, as the visitors levelled via a close range header from Brazilian international Casemiro as United failed to clear their lines.
Yet the Hammers persevered and earned their reward as the game entered its final minute. Danny Ings, another second half substitute was felled inside the penalty area although play was initially waved on - until the VAR team urged referee David Coote to review the incident.
Having done so, and much to Man Utd's chagrin, the official pointed to the spot - and up stepped Bowen to fire home the winning penalty which lifted West Ham above Manchester United into 13th spot in the Premier League.
You may watch extended highlights of the game below.
West Ham Utd: Fabiański, Wan-Bissaka, Mavropanos (Todibo 46), Kilman, Emerson, Rodríguez (Cresswell 90+3), Álvarez, Paquetá (Souček HT), Soler (Summerville HT), Bowen, Antonio (Ings 71)
Subs not used: Areola, Coufal, Irving, Luis Guilherme
Goals: Summerville (74), Bowen (pen 90+2)
Booked: Paquetá, Mavropanos, Summerville, Emerson, Cresswell
Manchester Utd: Onana, Dalot, de Ligt, Martínez, Mazraoui (Lindelöf 84), Eriksen (Zirkzee 79), Casemiro, Rashford (Amad 59), Fernandes, Garnacho, Højlund
Subs not used: Bayindir, Evans, Ugarte, Wheatley, Amass, Fletcher
Goals: Casemiro (81)
Booked: de Ligt
Possession: WHUFC 43%-57% MUFC
Shots/on target: WHUFC 12/3-18/5 MUFC
xG: WHUFC 2.99-2.63 MUFC
Referee: David Coote
Tickets sold: 62,474
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