Incredibly less than 16 hours after his arrival in Thailand, Rooney was on his way home again after scans revealed he had torn his hamstring.
United immediately took the decision to send the 27-year-old back to the UK - even though he had only landed that morning.
It means Rooney will miss visits to Australia - including a planned trip to State of Origin, Japan and Hong Kong - in addition to United's tour opener against Singha All Stars in Bangkok on Saturday.
"Gutted to have picked up a niggling injury especially as training was going so well, heading back to Manchester to have it assessed fully but early indications are nothing to worry about," said Rooney on his Facebook page as he headed for the airport.
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.
The incident came 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.
It is bound to trigger conspiracy theories given it comes against the backdrop of uncertainty over his United future.
Moyes repeated again yesterday 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 later today as they continue preparations for the new campaign.
Arsenal have also been linked with the forward, as have Paris St Germain.
With Robin van Persie not due to link up with his team-mates until the get to Australia on Sunday, Shinji Kagawa resting for an additional week after the Confederations Cup and Javier Hernandez not involved in the tour at all, it leaves Moyes with only Danny Welbeck as an orthodox striker ahead of tomorrow's game.
It completed a difficult 24 hours for the new United boss, who is facing up to the probability that one of his major transfer targets, Thiago Alcantara, has snubbed a switch to Old Trafford in favour of linking up with former boss Pep Guardiola at Bayern Munich.
"Yes," Guardiola told reporters in Munich earlier, when asked if he was interested in Thiago
"I don't know what's going to happen. We have to wait. But I know him very well. He is a super, super player."