The right-hander has been troubled by shoulder soreness for the last several years, but the problem has been much more acute this season, with McCarthy missing time on three separate occasions.
McCarthy again had a flareup of shoulder pain Saturday and was scratched from Sunday's start vs. the Giants. He appears headed back to the disabled list, having been on the 15-day DL from May 20 to June 2 already this season.
The pitcher said that his issues are beyond frustrating. MRIs have shown that his shoulder is fine structurally, but nothing has been able to keep the pain from reappearing.
"The last two weeks have been about as mentally harrowing as I've had in my career," McCarthy said. "Dealing with pain every day is miserable, and the feeling of knowing if I can get out on the mound, I'll be fine, but it's just not doing it. The doctor's not telling you your arm looks like a gun went off inside it. They're saying it's fine and healthy, and it just doesn't feel right.
"You obsess over the pain, and the whole thing is just kind of a big stressful game that hasn't been fun."
As McCarthy pointed out, his performance when he has been able to pitch only adds to his and the A's distress. The 28-year-old has been pitching as well as in any point in his career, recording a 1.96 ERA while in the midst of a career-high six-game winning streak.
The plan for McCarthy's shoulder soreness remains the same as it's been -- rest for now, perhaps until after the All-Star break, and then try and build strength back up -- but while he's been able to recover in the short term, the right-hander hasn't been able to decipher a lasting solution.
The competitiveness in McCarthy tells him to pitch through the pain. He knows, though, that he has to sit out so as not to jeopardize his health or the team's success, even though "it's not easy being honest with yourself [about] when it's OK, when it isn't."
McCarthy said he even feels jealous of his fellow pitchers, all of whom are able to throw without worry every day.
"That's all I really want to do right now," McCarthy said. "There is that powerlessness of I feel like I've done everything we could do, tried different routes, and it just kind of seems to keep coming back without anything serious being in there at all.
"Nobody can look at me and say, 'This is why, let's address this,' and that's the way I like to handle things. You just kind of want to pull your hair out."
For now, it appears that right-hander A.J. Griffin will be called up to start in McCarthy's place Sunday, as Griffin was scratched from his start for Triple-A Sacramento. A corresponding roster move will have to be made since Griffin isn't on the 40-man roster, but all indications are he will be brought to Oakland.
McCarthy's long-term future is less clear.
Ben Estes is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.