The day Hakim Ziyech and Timo Werner officially became Chelsea players ended with Frank Lampard seeing that his side will continue to fall short until they sort out their accident-prone defence. Reality bit hard when Andriy Yarmolenko cut inside to seal a crucial win in added time for West Ham. Chelsea were a mess at the back throughout this chaotic London derby and while Lampard has sharpened his attack before next season, he would be wise to sign a top centre-back this summer.

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There is no doubt Werner and Ziyech will make Chelsea more potent up front. While they will improve an attack that still flatters to deceive, it is debatable whether Lampard should continue his pursuit of Kai Havertz if signing the Bayer Leverkusen attacker means he will not have funds for defensive reinforcements.

This was a calamitous display from Chelsea, who wasted an opportunity to move five points clear of Manchester United and replace Leicester in third, and another reminder they lack the balance to rival Liverpool and Manchester City.

With Kepa Arrizabalaga unwilling to ease the pressure by coming off his line to claim set-pieces, the defensive frailties were glaring. Antonio Rüdiger and Andreas Christensen were bullied by Michail Antonio and the final blow came when Marcos Alonso failed to track back to deal with a West Ham counterattack, allowing Yarmolenko to win the game.

The way Rüdiger was left exposed against Yarmolenko will strengthen Lampard’s determination to replace Alonso with the Leicester left-back, Ben Chilwell. Yet there were deficiencies across the back four and West Ham exploited them well, using Antonio’s physicality to good effect in the opening period before attacking clinically in the closing stages.

It was a huge win for West Ham, who rose three points above 18th-placed Bournemouth after completing their first double over Chelsea since 2002‑03. Antonio was outstanding up front, scoring once and creating the winner. Jarrod Bowen caught the eye with two assists and the return of Angelo Ogbonna helped, giving West Ham more assurance at the back.

David Moyes was entitled to feel pleased with himself after Yarmolenko came off the bench to score the decisive goal. It was a solid performance from West Ham, who will be confident about staying up after limiting Chelsea to few clear opportunities in open play.

Chelsea did not create much despite dominating possession in the opening period. They weaved pretty patterns on the edge of the West Ham area and Christian Pulisic bent a shot just wide after a dummy from Tammy Abraham, but picking the lock was far from straightforward.

Declan Rice was impressive for West Ham in midfield, using his positional sense to cut out through balls and his long legs to make important tackles, while there were reminders Chelsea are always liable to gift their opponents a chance.

West Ham’s equaliser on the stroke of half-time was a case in point, Rüdiger getting himself in a tangle and slicing the ball behind for a corner. The lack of composure under pressure was staggering and Chelsea paid the price when Bowen swung a corner to the far post, where Tomas Soucek rose above César Azpilicueta before heading in his first goal in English football.

Soucek has given West Ham greater presence since arriving on loan in January and the Czech midfielder was unfortunate to have a goal ruled out from another Bowen corner when the game was goalless. Arrizabalaga, rooted to his line again, was relieved that a VAR review showed that Antonio was lying in an offside position.

Chelsea opened the scoring shortly after that reprieve, Willian converting his second penalty in a week after Issa Diop’s foul on Pulisic. Yet vulnerabilities lingered. There was complacency at times, particularly when Arrizabalaga played his team into trouble with a risky pass. The goalkeeper was lucky Antonio shot wide.

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West Ham went ahead in the 51st minute, Pablo Fornals flickering into life on the edge of the area and finding Bowen, who beat Alonso before fizzing in a low centre for Antonio to score from close range.

The question before kick-off was whether West Ham would have enough bite without Sébastien Haller leading the line. Yet Chelsea never looked comfortable, even after Willian hauled them level with a wonderful free-kick that bounced in off the post.

Lampard had made attacking changes, introducing Mason Mount for Mateo Kovacic, but Chelsea were too sloppy. Abraham and Ross Barkley disappointed before being removed and Chelsea lost their shape in midfield as they chased a winner, allowing Antonio to jab a pass through to Yarmolenko. The winger cut on to his left foot and an emphatic finish left Lampard with much to ponder.