The Dutch striker, who has scored seven Premier League goals so far this season, has missed the Red Devils' last four matches.
"I don't know [when Van Persie will return]," United's official website quotes Moyes as saying. "We've been trying to get him back for a few weeks.
"But he is not ready yet. It's his groin. It's a sore groin injury and nothing desperately serious."
United are currently ninth in the league table with 22 points from 14 matches.