Myers leads the major leagues with 24 home runs allowed and is coming off the shortest start of his career with the Phillies.
Myers agreed to be optioned to Triple-A Lehigh Valley on Tuesday.
Myers, 3-9 after giving up five runs and getting knocked out without retiring a batter in the third inning in Friday night's 8-7 loss at Texas, has a 5.84 ERA.
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."
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."
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.
"I can't say enough about Brett's willingness to accept this assignment," general manager Pat Gillick said. "He understood and is eager to get back on track again. He is struggling right now, but he is a quality major league pitcher who we are going to need in the second half of the season."
Myers is 62-56 in his seven-year career, 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 also were 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.
