Myers also opened the season last year, but moved to the bullpen in mid-April when injuries began to decimate the relief corps.
"We're trying to set up our rotation the way we want," said Manuel, whose team clinched a playoff berth last year with Hamels' emergence as an ace and Myers at the back end of the bullpen.
"I did talk to Hamels; I felt like I should talk to him and he seemed like he definitely was OK with it," Manuel said. "I told him he was our No. 1 last year and he had a big year, but that's the way we wanted our rotation to line up."
Philadelphia was scheduled to play Florida State on Tuesday night but the exhibition game was rained out.
Myers made just three starts last year. He struck out nine in 7 2/3 innings in last year's opener against the Atlanta Braves, picking up a no-decision. He worked 7 2/3 innings total in his next two starts, allowing a combined 13 earned runs in two losses.
Hamels earned his first All-Star berth last year and became the quickest Phillies pitcher to reach 15 wins since Curt Schilling in 1999.
Despite missing a month with a mild left elbow strain in mid-August, the 24-year-old lefty went 15-5 with a 3.39 ERA and struck out 177 batters in 183 1/3 innings in his first full season with the Phillies.
Myers said he was surprised and honored by the news.
"Hamels had a great year last year; it's going to be hard for me to take over the No. 1 spot when he had that year last year," he said. "I'm going to have to work that much harder to earn that spot."
Prior to last year, when 48 of his 51 appearances came out of the bullpen, Myers was used exclusively as a starter. In six seasons, he has compiled a 59-47 record with a 4.34 ERA. In 2005 and 2006, he finished in the top five in the National League in strikeouts -- fifth in 2006 with 189 and third in 2005 with 208.
Myers also threw the last pitch of the regular season at Citizens Bank Park in 2007 when the Phillies clinched their first division title in 14 years on Sept. 30 against Washington.