The England striker, who has been sidelined for several weeks, will miss Saturday's home Premier League game against Wolverhampton Wanderers and Wednesday's derby at Manchester City.
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"He is going on Saturday and will return the following Saturday," Phelan told a news conference that he was attending in place of manager Sir Alex Ferguson who was ill with a virus.
"It might be in the interest of him and the club to have a change of scenery. He has had a lot of attention lately so we are going to give him the best possible opportunity that we can to be in good shape when he returns."
Phelan said Rooney would be under the supervision of United's medical staff during his week at a training facility in Oregon, on the west coast of the United States.
"He needs the conditioning, he can go there without the attention he would get around here," said Phelan.
"If that goes according to plan, then he's available when he returns."
Portugal winger Nani misses out with a groin injury he picked up in Tuesday's 3-0 Champions League victory at Bursaspor, while midfielder Ryan Giggs is a major doubt having just returned to training after an injury.
There was even better news for midfielder Darren Fletcher, though, who is in contention for a place on Saturday after showing "positive signs" of recovering from his ankle injury sustained midweek.
"We're starting to get a rhythm," said Phelan.
"We feel as if we might have got over these lapses of concentration and we are getting more of a settled unit now and that is proving in the performances and the results that we get."
