Myers agreed to be optioned to Triple-A Lehigh Valley on Tuesday.
Myers, 3-9 after giving up five runs in two-plus innings in Friday night's 8-7 loss at Texas, has a 5.84 ERA, sixth worst in the majors among qualifying pitchers.
Phillies manager Charlie Manuel said he spoke with Myers on Monday about accepting the assignment to work on his delivery. Myers, who has more than five years experience in the majors, had the right to decline the assignment and asked for a day to consider the move.
The Phillies are 1-11 in the last 12 games started by Myers, who is 1-8 in that stretch.
Myers will start Wednesday night against Scranton/Wilkes-Barre.
"We don't look at it as a demotion," pitching coach Rich Dubee said. "We look at it as the means to getting his delivery right. He's a guy we need."
The Phillies announced after Tuesday night's game left-handed reliever R.J. Swindle would be called up from Lehigh Valley on Wednesday.
Dubee said Myers needs to experience success against minor-league hitters to boost his sagging confidence.
"I think probably confidence-wise, he's as low as I've seen him," Dubee said.
General manager Pat Gillick, who spoke with reporters in a telephone interview from Philadelphia, said the team was not concerned that more struggles in the minor leagues would only hurt Myers' confidence.
"I think it's the right move," Gillick said. "To me, there's not a negative involved in this."
The move surprised observers who expected a less dramatic move for Myers. There had been speculation Myers would perhaps miss only one start and make a temporary move to the bullpen.
"I don't think it's drastic at all," Gillick said. "I think it's a very constructive move."
Manuel said Myers, who saved 21 games in 2007 before returning to the rotation this season, will remain a starter when he returns.
"Mentally this is going to give him a blow," Manuel said. "He needs to get sharp. He needs to get his confidence up."
Gillick said there is no timetable for Myers' stay in the minors.
"It could be two or three starts, it could be four starts," Gillick said. "I don't know."
Gillick said Myers will have less pressure in the minor leagues to work on changes suggested by Dubee.
"Rich has really diagnosed the problem and Brett has been slow to come around to really introduce the suggestions into his windup and his delivery," Gillick said.
"So hopefully not being at the major-league level and not having the pressure to win, that he can work on the things he has to work on, which is ... location of his fastball. If he learns to control his fastball, he could be back here very quickly," Gillick said.
Manuel said the team will replace Myers on the roster on Wednesday. Dubee said Cole Hamels will move up to take Myers' scheduled start in Atlanta on Thursday.
Myers is 62-56 in his seven-year career and won at least 10 games in four straight seasons before being moved to closer last season. He struck out 208 batters and had a 3.72 ERA in 2005.
The Phillies were also without right-hander reliever Clay Condrey for Tuesday's opening game of a series against Atlanta. Condrey left the team to be with his wife, who is having a baby.
Manuel said Condrey is expected to return to the team Thursday.