The infielder was a late scratch from the starting lineup at third base for Wednesday's 8-1 defeat because he had trouble holding the bat with his left hand.
McDonald is not certain what the problem is, but he first felt it while in the on-deck circle in the first game of a series with the Yankees on Sept. 3.
The discomfort started in his shoulder and worked down to his hand. There was concern it could have been an infection.
"It was on the inside of my arm," McDonald said. "Something was going on, and I don't have an explanation for it, when I did it or what happened. The fluids probably drained out of my shoulder and settled in my hand and made it tough for me to hold onto the bat. I don't have any muscle weakness. It's weird."