McDonald has fallen off considerably since a hot first half. The 27-year-old right-hander has allowed 14 runs in his last three starts -- a total of 12 innings pitched. He walked four batters and allowed four runs in just 3 2-3 innings in a loss to the Chicago Cubs on Friday.
Manager Clint Hurdle said Sunday that McDonald would become available out of the bullpen around Tuesday. McDonald is 3-5 with a 7.08 ERA in the second half. He went 9-3 with a 2.37 ERA in the first half.
Right-hander Kyle McPherson will start in McDonald's place Wednesday against Milwaukee. The 24-year-old has 11 strikeouts in 11 2-3 relief innings this season.