"Robin Van Persie is out for a month. He picked up a thigh strain in taking a corner kick which led to the goal (against Shakhtar Donetsk on Tuesday)," Moyes told a news conference ahead of Sunday's game at Aston Villa.
The Dutchman, who has scored seven goals in 11 Premier League matches this season, returned from a groin injury in last weekend's 1-0 home defeat by Newcastle United.
His absence will be a major blow to Moyes as United try and repair the damage of their poor first third of the season which has left them in ninth place, 13 points behind leaders Arsenal.
"It's unfortunate because I think if you look at our record with Wayne Rooney and Robin in the team, it is actually very good and we have not had them for quite a period recently," Moyes said.
(Reporting by Martyn Herman, editing by Ed Osmond)
