Ibrahimovic tore his anterior cruciate ligament against Anderlecht in April in the Europa League.
Dr Freddie Fu, the specialist who operated on him, wants the striker to have as much recovery time as possible.
"He is very strong but soccer is still a combative sport," he said. "As a doctor, of course I want to see him return as slow as possible, only when he is fully fit for the game.
"There have been many cases of a player returning prematurely with bad consequences.
"I have no worry about his injury as it always takes time to fully recover."
The doctor, based in Pittsburgh, added: "In fact, the EPL has just started and I am sure Manchester United would not need the player urgently for any crucial game either in the league or European competitions."