The striker, who has been West Ham's best player since Project Restart last season, returned from a hamstring problem against Aston Villa on Monday but has now suffered another problem, with the club concerned it is a separate issue and not simply a recurrence of the same injury.
David Moyes faces former club Manchester United on Saturday but will have to do so without his talisman, with Sebastien Haller set to return to the starting XI.
"Mick is not going to be available for the game," Moyes said. "We're not sure how long he will be out for, we hope it’s not too bad but he won’t be available for tomorrow’s game.
"Andriy Yarmolenko has trained the last few days. We are not sure how quickly he can fully recover,
some are quick and some are longer but the good news is he is back, he’s fit, he’s healthy.
"He just has a bit of a problem with his hamstring. It’s not the same injury, we think it is something slightly different so that’s good news, we are hoping he will not be out for too long."
Haller has been back in form of late and scored the winner against Sheffield United last month, and Moyes is now looking to him to step up.
“Seb was beginning to run into form and you could see that he was getting confidence from his games,” said the Hammers boss. “So we are looking to him now and hoping he can get the goals he has had at his previous clubs in previous seasons.”
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