Manuel Pellegrini says Marko Arnautovic is fully focused on playing for West Ham and has not been distracted by missing out on a big-money move to China.
The West Ham forward, who missed out on a January transfer to China, is going through a goal drought after failing to score in the last eight games, and has been criticised for under-performing in recent weeks.
Home fans have been unhappy with the Austrian's form, and booed him, and the team, during the 2-0 home defeat to Everton as the Londoners missed out on the chance to go seventh in the Premier League table.
Speaking ahead of the difficult trip to face Chelsea at Stamford Bridge on Monday night, live on Sky Sports, Pellegrini defended Arnautovic saying the forward needed support not criticism.
He said: "I don't think anybody can be happy. The fans are not happy, Marko is not happy but this happens to players, they are not in their best moment.
"They need support more than punishment.
"I am always talking with Marko, who wants to do things better, and he is absolutely involved with our team. What happened about China in December is over.
"Someone thinking that Marko doesn't want to be here or doesn't want to play at West Ham, they are wrong.
"He is not in a good moment but supporting him, he will return.
"He doesn't have the same support of the fans that he had six months ago but with work and the right mentality then he will return."
Pellegrini described West Ham's defeat to Everton as the worst performance since he took over the club, but says the season is not over and insists his team must put the bad result behind them.
He said: "Of course nobody was happy after the game, but we will try to finish this season as high as we can.
"The season is not over. Some might be happy we are not fighting relegation but our mentality is to fight more.
"I am not unhappy with the season with so many injured players we have had, I don't think this is a bad season."