West Ham assistant manager Neil McDonald said the team just could not cope with the amount of absent players in Saturday's 3-0 defeat at Arsenal.
The Hammers were without James Tomkins, who has a dislocated shoulder and Enner Valencia, who cut his toe after standing on a broken teacup.
"When you are missing vital players through injury, to try to adjust and playing one of the best players in the league becomes very difficult," McDonald said.
"Even the goalkeeper has dislocated his finger before the game [in the warm-up], but Adrian has been a brave lad and played on with a very good performance, making some great saves.
"The game plan was to stifle Arsenal as much as we could, but we did not pass the ball well enough until the second half and then we conceded two goals at the end which is disappointing."
West Ham boss Sam Allardyce passed up on his post-match media duties to try to refocus the team, McDonald reported.
"The players are really feeling sorry for themselves and Sam is trying to pick them up, but they have given their all," said McDonald.
"We have played five of the top six teams and the performances have been good overall, if disappointed with the results, but we have to look forward and be positive."