Myers, placed on the 15-day disabled list Friday, will have surgery on his right hip and may miss the rest of the season.
Bastardo, 23, is 3-2 with a 1.89 ERA in 11 games (seven starts) with Lehigh Valley and Double-A Reading. In two starts since being promoted to the IronPigs, Bastardo struck out 12 while walking just three and allowing 11 hits in 13 innings.
"We've had some real good reports on him," Phillies manager Charlie Manuel said Friday, before the decision was announced. "He's had some real good outings since he's been in Triple-A."
The Phillies will officially make the roster move before Tuesday's game in San Diego. Once Bastardo joins the club, Philadelphia will have four southpaws in its rotation -- Bastardo, Cole Hamels, Jamie Moyer and J.A. Happ -- but that does not bother Manuel.
"I'm not worried about that," Manuel said before Saturday's game. "I had four one time in Double-A baseball, and that might be one of the best pitching staffs that I had for that league. I liked it."
David Gurian-Peck is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.