West Ham manager Slaven Bilic says his side had "too many passengers" in their 4-1 Premier League defeat to Swansea City on Saturday.
Despite starting strongly, West Ham went in 2-0 down at half-time in the penultimate match at Upton Park, before Ki Sung-yueng added a third for the visitors.
A Stephen Kingsley own goal pulled one back for Bilic's men, but Bafetimbi Gomis' late strike added gloss to an unexpected victory for a much-changed Swansea line-up.
The result deals a severe blow to West Ham's chances of finishing in the top four and ends a run of 15 matches unbeaten at home in the league.
"We have to congratulate them and they deserved to win," Bilic said. "In basic situations we were second best. We had too many passengers.
"We had a great opening 20 minutes and everyone was waiting for us to score. But they took the lead and added another one.
"They put it to bed with the third goal. The way we defended was not good enough today and that is the biggest reason behind why we lost."
Sixth-placed West Ham host fifth-placed Manchester United on Tuesday in the last fixture to be played at their famous ground, and they will be hoping to reduce the gap between the sides to one point heading into the final weekend of the season.
"We need to react now," added Bilic. "We had a great unbeaten run at home and set the bar high.
"We have to bounce back and it will be a memorable night."