First-half substitute Arthur Masuaku’s unexpected strike beat Édouard Mendy three minutes from time, to condemn Chelsea to their second league defeat of the campaign, while these three points cement the Hammers fourth-place spot for another week at least.
The Blues had made four changes from the team that had won at Vicarage Road in midweek to maintain their lead at the top of the table, as Jorginho, Hakim Ziyech, Thiago Silva and Reece James earned recalls in place of Trevor Chalobah plus substitutes Saúl Ñiguez, César Azpilicueta and Christian Pulisic.
While the visitors were enjoying all of the possession, the sheer weight of the Hammers bodies in the final third meant that all Thomas Tuchel’s side had to offer during the opening 25 minutes were James’ low, long-range drive that Łukasz Fabiański got down to gather and a poorly-directed Kai Havertz header that the Polish stopper held comfortably.
Chelsea’s lack of cutting edge was clearly frustrating Jorginho, who was booked for dissent after being denied a foul but the Italian Euro 2020 winner was all smiles on 28 minutes, when Mount floated a corner towards the edge of the six-yard box, where Thiago Silva ghosted between Michail Antonio and Diop to send a thumping header crashing into the net via Fabiański’s left-hand post.
Having just given his team the lead with his second goal of the campaign, the Brazilian then saved his side at the other end, clearing off the line as Coufal thought that he was about to score his first-ever goal after the Hammers had immediately broken upfield, with the Czech wing-back letting fly with a low, angled 18-yard drive.
With Mount, Ruben Loftus-Cheek and Andreas Christensen all restricted to long-range efforts, Moyes men still found themselves well and truly in the contest and, five minutes before the break, Jorginho’s careless, underhit backpass left Mendy in all sorts of trouble.
Having been now caught in possession by the tough-tackling Bowen, the Chelsea ‘keeper desperately bundled him to the ground and, with referee Andre Marriner instantly pointing to the spot, Lanzini made no mistake, lofting the consequent penalty under the left-hand angle as Mendy dived in the opposite direction.
Now on level terms, all the Hammers needed to do to was get to the interval all-square but with just a minute of the opening period remaining, Declan Rice yielded possession to Marcos Alonso who released Ziyech down the left-flank.
The Moroccan soon sent over a deep cross to Mount, who escaped the attentions of Johnson before expertly placing a technically-brilliant 10-yard, side-foot volley inside the base of the flailing Fabiański’s near post.
Trailing once more, West Ham’s agony was compounded, when Johnson failed to continue and Masuaku tellingly stepped from the dug-out for the final seconds in which Zouma and Havertz both needed treatment, too.
That late clash resulted in the half-time retirement of Havertz, who was replaced by Romelu Lukaku and, while all attention was on the fit-again, record £97.5 million striker, it was a Hammers marksman who quickly came under the Stratford spotlight.
Indeed, just 10 minutes into the second period, the Hammers drew level after Souček and Antonio combined down the right channel before the ball broke to Coufal, who prodded onto Bowen and he bagged his fourth goal of the campaign with a stunning low 18-yarder that scorched through Christensen’s legs as it steamed through the chilly East End air, beyond Mendy.
While Chelsea continued to create bits and pieces in and around the West Ham area, Moyes boys defended manfully, while still looking to counter and, sure enough, with just three minutes remaining they netted their wonderful, albeit bizarre, winner from an unlikely goalscorer standing in an equally unlikely position.
Receiving his quick throw-in back from Antonio deep in the left-hand channel, Masuaku dwelt patiently as he took a few steps towards the edge of the Chelsea penalty area. There may have been a couple of Hammers at the far post but the DR Congo international had other ideas and let fly an incredible swerving shot towards the near post, which the diving, deceived Mendy could only palm into his own net.
The modest Masuaku covered his ears as team-mates and fans alike raucously celebrated only his second goal in 116 outings in a Hammers shirt and his first-ever in the Premier League.
Quite simply, there was no time for any crestfallen Chelsea comeback, while this Hammers valiant victory secures fourth-place for another weekend during which the beaten Blues will be looking nervously over their shoulders given both Manchester City and Liverpool now have that coveted top-spot in their sights.
HAMMERS: Fabiański, Diop, Zouma, (Fornals 70), Dawson, Coufal, Johnson (Masuaku 45), Rice, Souček, Lanzini (Benrahma 85), Bowen, Antonio.
Unused subs: Areola, Yarmolenko, Vlašić, Noble, Král, Ashby.
BLUES: Mendy, Christensen, Rudiger, Thiago Silva, Alonso (Pulisic 72), James, Ziyech (Hudson-Odoi 63), Jorginho, Loftus-Cheek, Mount, Havertz (Lukaku 45).
Unused subs: Arrizabalaga, Werner, Ñíguez, Barkley, Azpilicueta, Sarr.