Cummins hasn't bowled since Australia's tour of the West Indies in July, with scans last week showing a stress injury in his back has not yet healed.
There were signs of some improvement with the 32-year-old now back running, but still some way off being cleared to return to play.
Officials believe they could know by next week whether Cummins will play at Optus Stadium, but insist it is far too early to know his availability for the summer.
Ideally, Cummins would likely need at least four weeks of bowling to be declared match ready, with the first Test in Perth now six weeks away and Scott Boland the likely man to replace him.
The second Test in Brisbane then begins almost a fortnight later, before a similar run into the third at Adelaide Oval.
"Speaking to Patty over the last few days as well, he's a lot more optimistic (for Perth)," McDonald said on Friday.
"Those who have been through this type of injury, these things ebb and flow. So a couple of weeks ago, did he think he was a chance? Potentially not.
"He was a little bit sore. Then you flip it forward two weeks, and then there's a lot more positivity coming from there.
"We've added some variables into his training in the last couple of weeks and had a positive result there.
"The key variable to add back in to get him ready for the first Test match would be bowling. We're a little bit off that at the moment, but there's some positive signs.
"We've got a bit to work through there. We're still optimistic, hopeful."
McDonald was also confident Cummins could find form quickly, believing the right-armer does not need much preparation.
But there are also risks involved with rushing the world's No.4-ranked bowler back.
Cummins will have gone four months between his last match and the first Ashes Test.
That could prompt officials to weigh up whether to bring him back via the Sheffield Shield or another form.
"All those things will weigh up with Pat once we get back to where we need to, which is the biggest variable to add in, which is bowling," McDonald said.
"My view on it would be (he needs) four, four-and-a-half weeks (of bowling). And myself and Patty have spoken about that type of time frame.
"You also start to bring in other risk factors (otherwise). If he's not skill ready. Or soft tissue injuries then becomes a real risk.
"And if you were to do a soft-tissue injury early in the series, like the Ashes, then it's a long way back in from where he's been."
Beyond his obvious value with the ball, Cummins' availability could loom as crucial mid-series with short turnarounds between the third, fourth and fifth Tests.
It would be unlikely Australia could get through those with the same three quicks, unless the first of those Tests in Adelaide is a shortened affair.
Australia will at least have allrounder Cameron Green back bowling unrestricted for the series.
Green only bowled four overs in his Shield return last weekend, owing to Western Australia in the field on back-to-back days.
The 26-year-old has indicated he will play three rounds before the opening Test, while also featuring in Australia's ODI series against India.
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