The outfielder tried to overcome an oblique strain with treatment from the Red Sox medical staff, but the aching has continued to nag him, forcing the team to place McDonald on the 15-day disabled list on Sunday.
McDonald, who said he couldn't pinpoint the beginning of the pain, agreed with the team's decision.
"It's better for the team [and] myself to get it 100 percent," McDonald said. "Just get it ready, instead of just letting it nag. I've been dealing with it for a few weeks now, just kind of getting by, but [I] got tired of getting 90 mph [pitches] blown by me, so I need to get it right."
A week ago, McDonald also took the mound for the Red Sox, pitching one inning in a 17-inning loss against the Orioles. When asked if the pitching triggered the aching, McDonald shook his head.
"My whole body was sore after that," he said. "Nah. Like I said, it wasn't one particular thing. It's been going for a while. We've been treating it, and it just hasn't gotten 100 percent. They want me to rest and get it right."
In 26 games this season, McDonald is hitting .179 with two homers and eight RBIs.