Wainwright, 37, strained his hamstring while running the bases in the fifth inning of Sunday’s start against the Cubs. The right-hander has pitched to a 4.46 ERA with a 61/31 K/BB ratio in 70 2/3 innings this season.
It is not clear yet who will take Wainwright’s spot in the rotation while he’s on the mend. Michael Wacha, who recently moved to the bullpen and Genesis Cabrera, who was recently demoted to Triple-A, are options, as are Daniel Ponce de Leon and Alex Reyes.
Helsley, 24, has been yo-yoed between the majors and minors a few times already this season. In 10 major league innings of relief thus far, the righty has yielded three runs on four hits and five walks with 11 strikeouts.