McDonald has fallen off considerably since a strong 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, after going 9-3 with a 2.37 ERA in the first half.
"As the games count down, we've got to look to give ourselves the best shot to win a game," Hurdle said. "Sometimes the realization something's been taken away, maybe that can help him -- a different kind of route, as far as focus, as far as attention."
McDonald admitted a lack of focus after taking the loss Friday. He matched his career-high with 171 innings pitched, set in 2011. Hurdle did not think fatigue was an issue.
"I didn't see the fatigue, but the execution was challenging to say the least," Hurdle said. "I think it's all part of the maturation process of a young man, a starting pitcher in the major leagues, and we've definitely seen two different sides of it this year, two different halves."
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.
"There are many people who speak highly of him throughout our organization," Hurdle said. "We'd like to see what he can do."
Pittsburgh snapped a seven-game slide Saturday and enters Sunday just two games behind St. Louis and Los Angeles for the second wild card spot.