Antonio underwent a scan after being forced off with discomfort against Manchester City, having scored in the 1-1 draw.
The forward has been plagued by hamstring problems during his career but Moyes on Friday insisted the 30-year-old’s latest problem was not "too bad" but would rule him out of Saturday’s trip to Liverpool.
"He’s been good for us, has played really well and is an important member of the team," Moyes said of Antonio. "When he played against Spurs he felt something, then against City and he felt it [again]. We don’t think it is too bad but it will keep him out of this game, that is for sure."
Only Harry Kane (12) has scored more than Antonio's 11 goals since the June restart and Moyes admitted the forward was as big a loss to West Ham as Virgil van Dijk, who could be sidelined the rest of the season with a knee injury, is to the champions.
"I think so but we have really been showing a great team ethic," he said. "We have been scoring goals from all different areas, we have been playing well as a team but I am sure Liverpool will be saying the same about that as well.
“Of course we will miss him, since we came back down from lockdown I think Michail has been as important as any striker in the league.”
"Obviously you’d rather play Liverpool without Virgil Van Dijk," Moyes added. "But probably the biggest thing going up to play Liverpool is without supporters, their support makes such a difference, with no crowd behind them hopefully that might make a difference as well."
Antonio's injury comes amid a flurry of muscle problems across the top flight and Chelsea boss Frank Lampard on Thursday said the Premier League had "dropped the ball" by voting not to continue with five substitutes this season.
West Ham were among the clubs firmly against the rule-change but Moyes said he understood Lampard's point and admitted the schedule was "far, far" too tough on players.
"It’s near enough the first injury we have had in the main, we have had a few others but we have been pretty consistent and kept the boys in good condition," Moyes said. "The medical team have been good and because of that, I won’t say we have been lucky because you need your players to be fit.
"I take the point that Frank Lampard is making [about increasing substitutes] because we are seeing a big number of injuries, the number of games players have to play at the moment is far, far too big and the scheduling is really difficult for many clubs. "
West Ham have taken eight points from four matches against Leicester, Wolves, Tottenham and City, raising calls for Moyes to be rewarded with a long-term contract.
"There isn't any pressure on anybody...when the time is right," he said. "Let’s wait and see, we have only won two and drawn two so let’s see what happens over the next few months."
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