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 tomorrow 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."
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.