Howard went on the disabled list on May 12, retroactive to May 10, with a strained left quadriceps. Friday was the first day he could be activated.
Howard was 2-for-6 with a homer and four RBIs in two rehabilitation games with Class A Lakewood.
He was in Friday night's starting lineup against the Atlanta Braves as the Phillies' cleanup hitter.
"That made our lineup look better to me," said manager Charlie Manuel.
Myers left Wednesday night's game at Florida clutching his right arm above the elbow. He had an MRI on Thursday, and the Phillies say he has a strained shoulder.
Myers was 1-2 with a 5.50 ERA and six saves in 21 games, including three starts, this season.
Myers said he hopes to be able to throw in eight to 10 days. This is the first time he has been on the disabled list in his career.
The Phillies are awaiting the return of closer Tom Gordon from an inflamed rotator cuff. Following Myers' injury, Manuel indicated Antonio Alfonseca, Ryan Madson or Geoff Geary would be used in save situations.
Philadelphia also assigned catcher Chris Coste to Double-A Reading and purchased the contract of left-hander Michael Zagurski from Reading.
Coste appeared in only four games with Philadelphia after he was recalled from Triple-A Ottawa on May 12.
Zagurski, 24, made a combined 18 appearances at Class A Clearwater and Reading. He had five saves and a 1.16 ERA for the two teams.