Hargreaves to remain in America for treatment
"He had a recurrence of the knee injury which is of great concern to us," the United website quoted manager Alex Ferguson saying after the Premier League club's 3-1 win over Celtic in Toronto on Friday.
"He will remain in America until he is ready to come back and train," added the manager.
"There is no point in leaving the people who are treating him and did the operation and then have to send him back again. It is pointless."
Hargreaves, 29, was outstanding for England in the 2006 World Cup finals but has barely played in the last two seasons for United since joining for 17 million pounds from Bayern Munich three years ago.
The Canadian-born player has had operations on both knees to try to cure a chronic tendinitis problem and is currently being treated in Colorado.