Paul Pogba hooked after 70 minutes as defeat confirms Man Utd's worst start to a season in 29 years.
Match Events
West Ham piled more pressure on Jose Mourinho with a 3-1 victory over Manchester United at the London Stadium.
Paul Pogba returned after his midweek spat with Mourinho, but it took just five minutes for things to unravel as record signing Felipe Anderson scored his first West Ham goal with a deft near-post flick.
A miserable first half for the visitors was compounded two minutes before the break when Andriy Yarmolenko's shot took a wicked deflection off Victor Lindelof to double the Hammers' lead.
Marcus Rashford halved the deficit just after the underwhelming Pogba had been substituted on 70 minutes, but Marko Arnautovic extinguished any hopes of a comeback with his fourth goal of the season four minutes later.
5' GOAL! West Ham 1-0 Man Utd (Anderson)
It's the worst possible start for United!
Noble threads the ball through for Zabaleta on the overlap. It's a tight call but the flag stays down, and the full-back play a low cross in for Anderson, who flicks the ball past De Gea for his first West Ham goal.
22' CLOSE! Lukaku's header hits the post
POST! McTominay's pass inside the full-back releases Young. He crosses first time and Lukaku gets across his man, heading the ball against the outside of the post.
43' OWN GOAL! West Ham 2-0 Man Utd (Lindelof)
When it's not your day, it's not your day!
West Ham double their advantage thanks to a huge slice of luck but they won;t care one bit! Yarmolenko tries to work some space in the penalty area before letting fly with a left-footed shot. The ball takes a massive deflection off Lindelof, which wrong foots De Gea, and the ball floats into the back of the net!
64' WHAT A SAVE from Fabianski to deny Fellaini!
Another lovely ball in from Young and this time it's met by Fellaini. The Belgium midfielder powers a header towards the bottom corner but Fabianski gets down low to his right to push the ball away to safety.
71' GOAL! West Ham 2-1 Man Utd (Rashford)
West Ham switch off at a corner and Rashford capitalises at the near post, turning the ball past Fabianski to give United a lifeline. Game on!
74' GOAL! West Ham 3-1 Man Utd (Arnautovic)
Where is United's defence? Noble slides the ball through for Arnautovic, who is one on one with De Gea. The Austrian provides the finishing touch to restore West Ham's two goal lead.
Jose Mourinho is absolutely furious with his defence on the sidelines...
Line-ups
West Ham XI: Fabianski, Zabaleta, Balbuena, Diop, Mauaku, Rice, Noble, Obiang, Yarmolenko, Arnautovic, Anderson.
Subs: Adrian, Ogbonna, Fredericks, Perez, Antonio, Diangana.
Man Utd XI: De Gea, Young, Lindelof, Smalling, Shaw, Matic, Fellaini, McTominay, Pogba, Martial, Lukaku.
Subs: Grant, Bailly, Mata, Rashford, Fred, Herrera, Darmian.