Manchester United midfielder Owen Hargreaves may have to be put his comeback from injury on hold.

The 28-year-old has had operations on both knees in the past year in a bid to cure tendinitis and there were hopes he would be back at the end of September.

But Hargreaves, whose last game was in September 2008, is out of the Audi Cup in Munich, which raises new concerns.

"We had hoped he would be joining us but he is still in rehabilitation," said United boss Sir Alex Ferguson.

"He has had major operations on both knees and we are not rushing that."

Hargreaves is currently in the United States being treated by renowned knee surgeon Dr Richard Steadman.

The England international will be desperate to reignite his United career, having only started 25 times for his club since a £17m move from Bayern Munich in July 2007.

It has been a frustrating spell at Old Trafford for the Canadian-born midfielder, who last played for United in the 1-1 draw with Chelsea on 21 September 2008.

Hargreaves had surgery on his right knee last November and on his left knee in January, with the procedures carried out under the guidance of Steadman.

The leading surgeon has previously operated on former Newcastle striker Alan Shearer, new United signing Michael Owen and ex-Red Devils forward Ruud van Nistelrooy, who is now at Real Madrid.

Hargreaves' rehabilitation has had to be a long and delicate process, with BBC Sport revealing in June that he could be forced to quit top-level football in the next two years if the chronic knee problem were to persist.

His condition, patellar tendinitis, can be described as inflammation of the tendon that attaches the quadriceps (thigh muscle) to the tibia (shin bone).

Hargreaves may not have joined the United squad in Germany but centre-half Nemanja Vidic and summer signing Antonio Valencia have travelled to Munich.

Vidic appears to have recovered from an ankle injury he picked up playing for Serbia in June, while winger Valencia - seen as a replacement for Cristiano Ronaldo, who has joined Real Madrid - needed to go back to Ecuador to gain a UK visa following his move from Wigan.

Ferguson also revealed that new signing defender Gabriel Obertan would miss the first three months of the season with an injury that was already known about when he signed from Bordeaux.

Owen Hargreaves has been forced to delay his return to training with the rest of his Manchester United team-mates.

The England midfielder made just three appearances early on last season before being forced to undergo surgery for tendinitis in both knees.

Red Devils boss Sir Alex Ferguson had already admitted that he did not expect the former Bayern Munich ace to return to action until September.

The Scot had hoped that Hargreaves would be able to link up with the Premier League champions to train with his team-mates as they prepare for the Audi Cup in Germany.

But that plan has now been scuppered and Ferguson was unable to put a time-scale on when the Canada-born 28-year-old would be back in action.

The United manager admitted: "Hargreaves will not play in this tournament.

"We had hoped he would be joining us, but he is still in his programme of rehabilitation.

"He has had major operations on both knees and we are not rushing that.

"He is young enough to come back once he is fully fit, but we are not rushing him."