McDonald was placed on the 15-day disabled list on July 2, retroactive to June 25, nursing a slight strain of his left oblique, which he thought was suffered in Kansas City in mid-May.
He continues to work out at Salt River Fields in Arizona, and D-backs manager Kirk Gibson said he may return to action as soon as Friday night.
"I talked to [McDonald] on the phone [Wednesday] and he feels better," Gibson said. "I think he's maybe going to play in the game tonight [in the Arizona League]."
McDonald did face a minor hiccup on his road to recovery, catching bronchitis during the All-Star break.
Gibson said the veteran shortstop is almost over the throat illness.
"I talked to him [Wednesday] and he was doing quite a bit of coughing," Gibson said. "He hasn't felt great. We'll see when he gets in the game how he feels."
Saito, who has yet to make an appearance with Arizona, has been out with a right calf strain and a respiratory illness.
He threw one scoreless inning with two strikeouts against the Arizona League Brewers on Tuesday, and will join McDonald in Salt River Fields for workouts.
"He's been throwing the ball much better and throwing more of his secondary pitches," Gibson said. "I sense he's feeling much more comfortable than he has all year."