Myers, who is 3-9 with a 5.84 ERA in 17 starts this season, had the right to accept or decline the assignment because he had five-plus years of experience in the Major Leagues. He accepted and will start for Lehigh Valley on Wednesday night against Scranton/Wilkes-Barre.
"He's run into mechanical problems and run into some things he's got to work out, and when he works them out, he'll be back here with the Phillies," general manager Pat Gillick said in a conference call with reporters before Tuesday's game against the Braves at Turner Field.
Gillick also said that no decision has been made regarding Myers' replacement in the rotation. That decision will likely come in the next 24 to 48 hours, he said.
Likely candidates to fill that rotation spot are reliever Chad Durbin and Minor Leaguers J.A. Happ and Carlos Carrasco.
Cole Hamels will start Thursday's series finale in Atlanta, which will give the Philadelphia ace another start -- three total -- before the All-Star break. He will be pitching on normal rest.
There was some thought over the weekend that the Phillies would shift Myers to the bullpen, where he made 48 appearances last season. But club officials decided Monday afternoon that Myers wouldn't consistently get in enough work to regain confidence and strength in his pitching arm.
"I just need to get back to a starter's mentality," Myers told Channel 10 News, who caught up with the pitcher at the Philadelphia airport. "That's one thing I think I was lacking this year. [Shifting back to the starting rotation] was tough for me."
Will going to Triple-A Lehigh Valley do the trick?
"I always thought I could work my way out of it [with the Phillies], but this will probably relieve some of the pressure I was feeling, being in first place with the team," Myers said. "Hopefully, this will fix everything."
Myers was told of the Phils' desire to option him on Monday, and the 27-year-old asked for a day to decide whether to accept or reject the assignment. He told the team of his decision late Tuesday morning.
"Brett felt like this was the best thing for him to do and we felt the same thing," manager Charlie Manuel said.
Myers had won just once in his past 13 starts, when he struck out a career-high 11 and allowed three runs on May 30 against the Marlins. In his last start, on Friday against the Rangers, Myers allowed five runs on five hits in two-plus innings, the shortest start of his career. Overall, the Phillies are 4-13 in his starts this season.
"The other day [Friday], he just didn't come out with a very good look about himself, so hopefully this gets him out of this environment and gets him down there where he can be stress-free and concentrate on making pitches, and hopefully getting his arm strength back and get his delivery back," Phillies pitching coach Rich Dubee said.
Myers, who started Opening Day, has recently had trouble locating his fastball and hasn't hit the outside corner as effectively as he once did. He has reported no discomfort in his pitching arm despite decreased velocity. The speed on his fastball has dipped from the low-90s to the upper-80s.
"I don't know if it's drastic," Dubee said of the team's decision to option Myers. "I think it was needed. It's good that Brett is willing to do it.
"We just didn't feel like he was going to get straightened out here, in this environment, pitching under a microscope all the time, and just all the stress that's going on in his game right now. We felt like it was something that he needed, and to his credit, he accepted it," Dubee added.
It is not known how long Myers' stint in the Minors will be, but club officials said he will be monitored on a day-by-day basis. Myers is expected to make at least three starts at Triple-A.
"I think it's the proper move, I think it's a constructive move," Gillick said. "I don't think about him going down there and struggling, I think about him going down there and improving his situation and getting back to the big leagues."
Ryan Lavner is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.