Slaven Bilic accused West Ham of carrying "too many passengers" after Swansea derailed their Champions League bid.
The Hammers, unbeaten at home since August and still harbouring slim hopes of a top-four finish, crashed 4-1 to an injury-hit Swans side with little to play for.
Throw in the fact that this was the penultimate match at the Boleyn, a ground Swansea had won at just once in its 112-year history - back in 1956 - and this was a result hardly anyone saw coming.
Yet goals from Wayne Routledge, Andre Ayew and Ki Sung-Yueng stunned West Ham inside 50 minutes.
Diafra Sakho pulled one back, with the help of a final touch from Stephen Kingsley, but fellow sub Bafetimbi Gomis killed West Ham off in stoppage time.
And Hammers boss Bilic said: "We were simply not good enough, the whole team. We had too many passengers relying on other players to do the dirty jobs."
West Ham, who move to the Olympic Stadium this summer, will bid farewell to Upton Park against Manchester United on Tuesday night.
However, what could have been a shoot-out for a Champions League spot now sees the Hammers attempting to cling on for a Europa League place.
Bilic added: "I keep saying the gap between us and the top four is bigger than the gap between us and Southampton and Liverpool below.
"That's the gap that worries me and that's the gap I'm still concentrating on.
"But we have a game on Tuesday and we are still in a brilliant position. We want to finish high. With two games to go we are talking about Europe. It's in our hands. It's there for us."
Swansea had scarcely ventured out of their half until, in the 25th minute, Kyle Naughton fizzed in a low cross for Routledge to apply the finish.
Six minutes later a carbon copy goal from the opposite flank saw Kingsley cross for Ayew, and another tap-in.
Five minutes after half-time Swansea made it three, Modou Barrow bamboozling Michail Antonio on the byline before cutting the ball back for Ki to slot home.
Sakho stabbed in a rebound, via the boot of Kingsley, to give West Ham a slim chance of a comeback but that was extinguished when Ayew teed-up Gomis for number four.
Swans boss Francesco Guidolin insisted it was not their best display of the season, but claimed the result could define their campaign.
"It was a good performance and now we can say it is not a bad season," he said.
"Now we can say it was for us a season with difficulties, but one we have the possibility to finish well."
Guidolin's future beyond next Sunday's clash with Manchester City remains uncertain - he has been linked with the Watford job while Brendan Rodgers continues to be linked with his.
But he added: " I think my job is a good job. I am happy for the final (part) of the season, but the future I don't know.
"I did my job with enthusiasm, with feeling, and I am happy for our fans."