Antonio has picked up a groin injury, and the West Ham wide man could be kept out for longer than first anticipated.
Antonio’s absence will be a blow to the Hammers, but Moyes is keen to reduce the impact his injury could have.
The Scottish boss now suggests that West Ham are in the market to sign a right-sided player in the coming weeks, as he looks to do business.
“Michail Antonio has not fully recovered from a groin problem. Our medical people believe it could clear up in a couple of weeks but if not it could be more serious,” Moyes said.
“We’re not sure yet but if it is the latter - and that would be a real shame - we will need to go out to try to make a suitable signing this month because we will need someone who plays down that righthand side.”
West Ham are thought to have money to spend this month, but they are yet to complete a signing.
The Hammers have been linked with a whole host of players, but getting deals over the line has proven problematic during the early part of the transfer window.
Moyes wants to add quality to his squad, as he fights to keep West Ham in the Premier League.
The Hammers are currently in 15th place in the top flight table, and sit just two points above the drop zone.