Rooney suffered the injury in training earlier on Thursday and scans have revealed the full extent of the damage.
"Following a scan this evening [Thursday] on a hamstring injury sustained in training, it has been decided Wayne Rooney should return home immediately for further assessment and rehabilitation," the club said in a statement.
The news is an unexpected development given United only landed in Bangkok on Thursday and went straight to their hotel for a short rest ahead of what turned out to be Rooney's single training session.
Sixteen hours later he is on his way to the airport to begin the marathon journey back to Manchester.
If Rooney is ruled out for a month it would mean he could feature in the Community Shield against Wigan at Wembley on August 11.
However, given his obvious lack of match practice, it is difficult to see how he could be involved in the following week's clash between England and Scotland, or start the Premier League opener at Swansea on August 17.
There was even a picture distributed of Rooney at Thursday's training session laughing with Patrice Evra that was distributed by United's own photographer, although the pair were sitting on a step and the forward was not wearing boots.
And amazingly, the incident comes just a day after manager David Moyes revealed United staff had told him Rooney had returned for pre-season training in his best shape for years.
The injury robs United of one of their most recognisable faces - with Robin van Persie not linking up with his team-mates until they reach Sydney - and the man who is more in demand for personal appearances than anyone else.
Yet it is bound to trigger conspiracy theories, given it comes against the backdrop of uncertainty over his United future.
Moyes repeated again Wednesday that Rooney is not for sale and that the 27-year-old would not be going anywhere.
However, there has still been no word from the player about his intentions, even though there has been plenty of opportunity to silence the rumour mill should he so wish.
Chelsea are said to be interested in purchasing Rooney and, ironically, Jose Mourinho and his players are also due in Bangkok tomorrow as they continue preparations for the new campaign.
Arsenal have also been linked with the forward, as have Paris St Germain.
The issue is not quite so straightforward from United's side either.
Rooney currently has two years remaining on his contract and is therefore at a point when negotiations over an extension would normally begin.
It is difficult to see them wanting to commit to another £250,000-a-week deal though given Rooney's reduced status, and the Red Devils would be sanctioning a significant reduction in potential transfer fee should they allow the contract to run down beyond its present level.
There is also the thorny issue of Sir Alex Ferguson's declaration last season that Rooney had demanded a transfer, something the player has privately denied.
It lends an air of uncertainty over United's tour, with Moyes due to speak tomorrow ahead of his side's opening game against the Singha All Stars on Saturday.