Lucas Paqueta’s spectacular long-range finish handed West Ham an early lead, but they were soon pegged back as Joao Pedro headed home after 15 minutes.
West Ham thought they had an immediate response when Niclas Fullkrug finished smartly, but VAR intervened to rule the goal out for offside.
From that moment, the game spun as Chelsea turned the screw, adding goals through Pedro Neto and Enzo Fernandez before half-time.
It didn’t get any better for West Ham after the break, as they conceded twice in the space of four minutes, with Moises Caicedo and Trevoh Chalobah capitalising on some calamitous defending from the hosts.
Chelsea could and arguably should have had more on a night when Graham Potter’s future as West Ham manager was thrown into serious doubt.
Sam Tabuteau assessed the action from the London Stadium…

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The results speak for themselves: West Ham have not played well across their first two Premier League games.
However, they have had spells in which they’ve been in both games, having started positively against Sunderland and Chelsea.
That, though, is perhaps the most damning thing about their start to the season, their propensity to concede in short, sharp bursts.
Against Sunderland, they conceded three goals inside 30 second-half minutes, while Chelsea scored three times inside 19 first-half minutes at the London Stadium and twice inside four minutes after the break.
The manner of the defeats is one thing, but the ease at which West Ham lose sight of their game plan and begin to play within themselves is worrying.
West Ham look devoid of an identity right now, and in times of trouble, they have nothing to fall back on.
Relegation talk is always premature at this stage of the season, but West Ham need to find some common ground and quickly.

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Paqueta shows his quality but…
Lucas Paqueta looked a shell of himself last season, burdened by the weight of an FA investigation into spot-fixing charges.
Cleared this summer by an independent regulatory commission almost two years after the investigation was launched, the Brazilian looked like the weight of the world has lifted from his shoulders early on tonight.
His goal, a rocket of a shot that flew past Robert Sanchez, was the sort of effort only a few players would even try to attempt.
He is a player of undeniable quality, but he has to do more to help his teammates out of possession.
Twice he lost out in a duel, and twice Chelsea scored to turn the game on its head.
Paqueta should have been stronger up against Marc Cucurella, opting to give the defender a nudge in the back rather than contest for a header. Cucurella, off balance, won the flick on, and Pedro headed home.
Eight minutes later, Paqueta was then eased off the ball by Chalobah for Chelsea’s second, allowing Pedro to cross for Neto to side-foot past Hermansen. A hint of a foul, perhaps, but Paqueta should have shown more desire to keep hold of the ball.
No one is doubting Paqueta’s ability, and he is more than capable of dragging West Ham out of the mess they find themselves in, but Potter’s whole ethos during pre-season was that he wanted a tight-knit group that worked hard for each other. In that regard, Paqueta let his teammates down.

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Potter looks out of ideas
Potter came out swinging in his press conference, determined to prove he and his players were capable of bouncing back from their defeat to Sunderland.
All that fire and passion for a performance as tepid as the one West Ham produced against Chelsea was indicative of the disconnect between where West Ham want to be under Potter and the reality of the situation.
After a positive start, West Ham sank without a trace, conceding five without reply as they fell to the foot of the Premier League table.
Potter, who admitted pre-match that he had spoken honestly with the squad in the wake of their defeat at the Stadium of Light, had the opportunity to get around his players at half-time and conjure up a reaction, yet somehow West Ham got worse, repeating the mistakes of last weekend.
West Ham conceded twice from crosses into the box against Sunderland, and it was clear they hadn’t learnt their lesson as Caicedo and Chalobah took advantage of some inexcusable set-piece defending to score Chelsea’s fourth and fifth goals, respectively.
Potter looked bereft of ideas on the touchline. Not everything that went wrong for West Ham was the Englishman’s fault, but the manner of his side’s collapse will worry him no end, and could ultimately cost him his job, if not tonight, then in the near future. This was the type of result that can hang over a season.