Manchester United slipped to a third Premier League defeat of the season, as the improving West Ham United proved clinical in front of goal in Saturday's lunch-time contest.
Felipe Anderson's fifth-minute flicked finish and a wickedly deflected shot by Andriy Yarmolenko, later chalked up as a Victor Lindelof own goal, put the Hammers 2-0 up for half-time.
Although substitute Marcus Rashford pulled a goal back with a sublime backheel to outshine Anderson's fancy effort, Marko Arnautovic almost immediately restored the two-goal gap to make the painful final score 3-1.
After being pegged back to a draw against Wolves the previous weekend, Jose Mourinho made four changes to his starting XI, which included handing a first start of the season to Scott McTominay.
The 21-year-old was joined by Ashley Young, who skippered the side, Nemanja Matic and Anthony Martial, who all featured in Tuesday’s midweek cup exit to Derby County.
It was West Ham that started the brighter of the two teams. Fabian Balbuena had an early effort on goal, but the Paraguayan’s header did little to trouble David De Gea as it was skewed wide of the target.
It was a difficult opening for Mourinho’s men. Marko Arnautovic very nearly doubled the hosts’ lead ten minutes later, but De Gea was quick off his line to beat the athletic Austrian to the ball.
United, though, soon started to grow into the game and had a big chance to level the scores midway through the first half. A beautifully weighted cross found the head of Romelu Lukaku, however the Belgian’s effort hit the post.
The hosts, though, ended the half brightly and doubled their advantage two minutes from half-time. Andriy Yarmolenko cut inside and the Ukrainian’s crack at goal clipped Victor Lindelof to deflect past the helpless De Gea and leave the Reds with a mountain to climb going into the break.
Having made no changes at half-time, Mourinho brought on Marcus Rashford for Victor Lindelof shortly before the hour mark as the Reds continued to search for a route back into the game. Jose's side very nearly found that route when Marouane Fellaini’s downward header from Young’s curling cross forced a brilliant save from Hammers keeper Lukasz Fabianski.
As the visitors continued to apply the pressure, West Ham still posed a threat on the break. The Reds were given a warning as a through ball by Anderson was just inches away from being slid into the goal by Arnautovic.
However, any hope of a late comeback was extinguished two minutes later. Mark Noble’s slide-rule pass found Arnautovic who went one-on-one with De Gea and confidently slotted the ball home and put the hosts out of sight.
West Ham: Fabianski, Zabaleta, Balbuena, Diop, Masuaku, Rice, Obiang, Noble (c), Yarmolenko (Snodgrass 72), F. Anderson (Diangana 90+3), Arnautovic (Antonio 83).
Subs not used: Adrian, Ogbonna, Fredericks, Lucas.
United: De Gea; Young (c), Smalling, Lindelof (Rashford 57), Shaw; Fellaini, Matic, McTominay; Pogba (Fred 70); Martial (Mata 71), Lukaku
Subs not used: Grant, Bailly, Darmian, Herrera.
Scorers: West Ham - Anderson 5, Lindelof own goal 43, Arnautovic 74. United - Rashford 71.
The Reds return to UEFA Champions League action on Tuesday night with a key home clash against Valencia. Kick-off at Old Trafford will be 20:00 BST.