West Ham manager Slaven Bilic says scans show Diafra Sakho was injured at the weekend.
Sakho was not in the squad for Saturday's 3-3 draw with Arsenal, and a message posted on social media sparked reports of a falling-out with his manager.
But that was denied in the aftermath of the game by Bilic, who revealed the striker had pulled out citing an injury.
And Sakho will not be fit for Wednesday's FA Cup quarter-final replay against Manchester United - the last cup tie at West Ham's Boleyn Ground.
"The situation with Diafra is very simple," Bilic said at his pre-match press conference. "He reported an injury after training on Friday. He was not in the squad.
"Yesterday he had a scan, which showed a little bit of a problem and he will be out for one week."
Andy Carroll, fresh from his hat-trick against Arsenal, will start up front as the Hammers aim to book a Wembley date against Everton later this month.
Bilic said: "It's the final FA Cup tie at the Boleyn Ground, it's Manchester United and it's the quarter-finals. It can't be bigger.
"We are fully motivated and it is a massive game of course. It is massive for us and it is massive for them."
And he repeated his weekend appraisal of Carroll's qualities, saying: "When he is fit and in form he is very special and gives you something, he is different.
"It is hard to mark him and hard to cope with him. For us he has done really good every time he is fit. He doesn't have to score, his presence on the pitch gives us another quality and dimension."