Manuel Pellegrini claimed 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 Bournemouth.
Austria international Arnautovic, the club's joint leading scorer with eight goals, is keen on a big-money move to China, with Shanghai SIPG having already had a £35million offer rejected.
Hammers boss Pellegrini said on Thursday that the 29-year-old would be allowed to depart, subject to a "good deal for both parties" being agreed.
Second-half goals from Callum Wilson and Joshua King condemned the Hammers to defeat at the Vitality Stadium after Arnautovic's replacement, Andy Carroll, squandered a golden chance with the game goalless.
"The reason is because as a manager you must choose the players that work better during the week," said Pellegrini, explaining his team selection.
"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.
"I never complain about players who are not here.
"We couldn't win and, if Marko Arnautovic didn't play, we have another player that must do it."
Spanish striker Lucas Perez was also left out the Hammers' 18-man squad.
Asked about that decision, Pellegrini replied: "Same as Marko Arnautovic – I choose the 18 players I think are better during the week."
Arnautovic waved to home fans when he was substituted during last weekend's win over Arsenal.
His absence was the major talking point ahead of Saturday's south-coast clash and resulted in the Hammers' only change, with Carroll given just a second league start of the season.
He failed to take the opportunity, blazing his side's best chance over the crossbar from a couple of yards with the goal gaping after Michail Antonio made a mess of the initial effort.
"When we started the second half, we had a clear option to score the first goal, maybe that will change the game, absolutely," said Pellegrini of Carroll's effort.
"Unfortunately for Andy and for Antonio before that they couldn't score, but it's just part of the game."
First win of 2019 ✅ First clean sheet of 2019 🖐️ A goal each for the front men 💪 Enjoy your Saturday night Cherries! 🍒 pic.twitter.com/yXXiVT05RZ — AFC Bournemouth (@afcbournemouth) January 19, 2019
Victory for Bournemouth was just their third from 13 top-flight games and ended a five-match winless run in all competitions.
Like Arnautovic, Cherries top scorer Wilson, who returned from a two-match injury absence, has been linked with a January transfer.
Wilson, the subject of speculation linking him with a move to Chelsea or to the London Stadium as a possible replacement for Arnautovic, underlined his value to Eddie Howe's side by powering home his 10th league goal of the season from the edge of the box early in the second period.
King tapped in David Brooks' stoppage-time centre to seal the welcome win.
Wilson was substituted due to injury 17 minutes from time and manager Howe said it was not a recurrence of his recent hamstring niggle.
"I think it's a problem with his knee, I've got no more detail that that," said Howe.
Speaking about rumours of Wilson leaving, Howe added: "I've not seen any reaction to the speculation, it is what is.
"I think Callum's been in the game long enough to know there's nothing you can do about it.
"He's just got to concentrate on his football and he's very much a team player – a vital part of what we do here.
"And he knew how important the game was for us as a group and he delivered a very good display."
