McDonald was placed on the 15-day disabled list on July 2, retroactive to June 25, nursing a slight strain of his left oblique, an injury he thinks he sustained in mid-May.
"I've come a long way. I don't want to put a [percentage] number on it," McDonald said. "I feel good. I'm getting better every day."
The 37-year-old took Saturday off but will return to action on Sunday. The D-backs originally thought his injury would keep him out for as little as a week, but the healing process has taken longer than the club hoped.
"Any time you have something like this, you don't know how long it's going to be, when it's going to be fully back," McDonald said. "I would have liked it to have gone quicker, but unfortunately, you have to make sure you're completely healthy before you come back to play. You see too many times when guys come back too fast. I'm not talented enough to play at half speed."
Moving forward, McDonald will continue to play in the Arizona League before the team decides its next move.
"It's certainly a good first step in my mind," manager Kirk Gibson said.
Even when the veteran feels ready to make a comeback, he knows the ultimate decision will belong to the D-backs, not him.
"Whatever plan they have in place for me is fine. They've got a lot of things going on besides trying to activate a guy as soon as he is ready," McDonald said. "It's completely up to them."