There was plenty of excitement about the prospect of Ward-Prowse’s set pieces after he signed for West Ham, and it took a mere seven minutes of his debut for that to be vindicated, as Nayef Aguerd rose above Conor Gallagher to head home from his corner delivery.
Chelsea incomer Nicolas Jackson also offered encouragement, although that was also accompanied by frustration as a marginal offside call robbed the frontman of a penalty before he sent two efforts over the crossbar.
Chelsea dominated after going behind and eventually got their reward before the half-hour mark. Carney Chukwuemeka showed quick feet to work space from Tomas Soucek and smashed the ball past Alphonse Areola, marking his second start of the season - which equalled his tally from the entire last league campaign - with his first-ever senior goal.
That pattern continued and resulted in a penalty when Raheem Sterling was brought down by Soucek, although Areola came to his side’s rescue when he outfoxed Enzo Fernández to save the Argentine’s penalty.
There was further Blues disappointment before the break with Chukwuemeka hobbling off, and that was accentuated when West Ham went ahead within eight minutes of the restart.
Ward-Prowse was the architect again, slipping the ball through for Michail Antonio to stand up Axel Disasi and fire his effort out of Robert Sanchez’s reach.
Aguerd made the Hammers’ task more difficult in the 68th minute, though, picking up a second booking for a rash tackle on Jackson having seen an early yellow for tripping Sterling.
David Moyes’ men still stood strong in the face of constant waves of Chelsea pressure to successfully grind out their first win before having relief towards the end.
In contrast to Ward-Prowse, Moises Caicedo had a nightmare debut and conceded a penalty that Lucas Paqueta calmly stroked home.
That added to the misery of a Chelsea side that awaits their first victory under Mauricio Pochettino and has now lost three of their last five PL games.
Flashscore Man of the Match: James Ward-Prowse (West Ham United)
