The England international is set for a four-week spell on the sidelines after damaging his hamstring while in Thailand, but David Moyes says his withdrawal is necessary
The wantaway striker had flown out to Thailand with the rest of the squad and was expected to feature in David Moyes' first game in charge against Singha All-Star XI on Saturday.
However, the 27-year-old suffered an injury during the first day of training and will fly home immediately from the three-week trip, with the club confirming he will need "further assessment and rehabilitation".
"Gutted to have picked up a niggling injury especially as training was going so well," Rooney wrote on his Facebook page.
"Heading back to Manchester to have it assessed fully but early indications are nothing to worry about."
United's medical team expect Rooney to be unavailable for four weeks, which has cast doubts over his participation in the club's Community Shield clash with Wigan and their opening Premier League game of the season against Swansea City.
Moyes, though, insists Rooney's withdrawal is necessary with the new season less than a month away.
"It's disappointing," Moyes told MUTV. "The day we left [Manchester for the tour] he just felt his hamstring tighten up right at the end of training and we thought there would be no problem. But after the long flight we decided we'd get him tested and scanned and have a look at things and there is just a small grade one injury there.
"He didn't train yesterday and we thought it would be worth just seeing what it was and there was a very slight mark on his hamstring. We won't take any chances - there is no point in taking him any further round the world so we've sent him back and he'll get treatment at the Aon Training Complex."