The Frenchman suffered the knee injury on Sunday against Manchester United, leaving the game in the 65th minute of the 2-1 loss at Villa Park.
Villa are fifth in the table, a point behind Tottenham Hotspur.
"When they are injured for a long time, it's the worst in football," Emery told reporters. "It's more difficult for the player, as a person, when they are injured ... more or less, in September or October he is coming back."
Villa play 12th-placed Fulham at Craven Cottage on Saturday.
