Top knee specialist Dr Richard Steadman told the Manchester United ace he will only learn if surgery has worked when he returns to action.
England midfielder Hargreaves has targeted a first-team return in December after more than a year out of the side.
Only then will he discover whether his knee complaint has fully cleared up.
He flew to Germany in July for a second opinion from specialist Hans Muller-Wolfahrt, who was his physio at Bayern Munich.
Hargreaves, 28, has been back at United for just over a week.
He has yet to start running again but he has been stepping up his gym work at the Carrington training ground.
Hargreaves is on a special 45-minute workout regime to regain the strength in his leg muscles and his general conditioning.
That is expected to be enhanced this week with some light running, but there is no pressure on him to return.
A United source said: "With this type of injury, it either heals or it doesn't. It's been great to have Owen back and he's really positive, but even he doesn't know the answers.
"Dr Steadman said he will only find out if the surgical procedures have been a success when he starts playing.
"Everyone is hoping for the best because he's such a great player."