Van Persie has not featured for the Premier League champions since scoring in the 1-0 win over former club Arsenal at Old Trafford on November 10 due to a groin strain.
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"I don't know (when Robin will return). We've been trying to get him back for a few weeks," the Scot said.
"But he is not ready yet. It's his groin. It's a sore groin injury and nothing desperately serious."
Van Persie's fellow striker Wayne Rooney will also miss Satuday's visit of Newcastle United having collected his fifth booking of the season during Wednesday's loss to Everton.
That capped off a tough night for Moyes, who tasted defeat against the club he left to join United in the summer.
"I'm disappointed to lose, we missed some opportunities and Everton took theirs. It's a sore one against your old team," he added.
"We have a long way to go, we will need to play some great football and win an awful lot of games."