Bastardo is likely to miss his next start, which is scheduled for Wednesday in Atlanta.
The 23-year-old Bastardo exited Thursday's game in the fourth inning with the injury.
Bastardo is 2-3 with a 6.75 ERA in five starts for the Phillies this year.
Philadelphia could pitch without a fifth starter until July 4 against the Mets at Citizens Bank Park. If the Phillies want to keep five starters through the All-Star break, they have options.
Right-hander Kyle Kendrick (4-4, 3.97 ERA) pitched Thursday for Triple-A Lehigh Valley, which means he could pitch for the club on normal rest. He allowed five hits and four runs in six innings against Indianapolis. It's also worth noting that right-hander Kyle Drabek (4-0, 2.43 ERA) pitched Thursday for Double-A Reading, giving up six hits and three runs in 7 1/3 innings against New Hampshire.
Philadelphia manager Charlie Manuel was not available for comment following Friday's loss. After a closed-door team meeting that lasted about 20 minutes, his office door was shut and he did not speak to reporters.
David Singh is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.