Manuel Pellegrini said Marko Arnautovic "has his head on another issue" after the player was left out of West Ham's squad for the 2-0 defeat at AFC Bournemouth.
Arnautovic, the Hammers' joint-leading goalscorer with eight goals, is keen to leave the club in January with clubs in China offering big money for the Austria international.
West Ham were abject in his absence, registering just one shot on target against a Bournemouth team who had lost three of their previous four heading into the game.
"The reason is because as a manager you must choose the players that work better during the week," said Pellegrini.
"Marko has his head on another issue so I thought it was not a good idea for him to play today.
"I don't think we need to continue talking about players that are not in the squad in this moment.
"We couldn't win and if Marko Arnautovic didn't play, we have another player that must do it."
Additional reporting by Press Association.
