The 24-year-old ruptured his anterior cruciate ligament (ACL) in Sunday’s 2-1 Premier League loss to Manchester United.
Villa attacker Emiliano Buendía and defender Tyrone Mings are also out with ACL injuries.
“When they are injured for a long time, it’s the worst in football,” Emery added.
“When a player is hurt, it makes things more difficult for them as individuals.
“Buendia and Mings have been out with injuries for the majority of the season.
“Now it’s the same with Kamara. Perhaps he will return in September or October.”
Diego Carlos, a defender, will be out of play for up to four weeks due to a hamstring injury suffered during training this week.
Villa are sixth in the table. travel to Fulham on Saturday.
