A back stress injury has cast doubt on the 32-year-old's participation in the Ashes series, which starts on November 21st, with the paceman yet to resume bowling in his rehabilitation period.
McDonald said those suffering from back stress injuries experience "ebb and flow" in their recovery, but that Cummins was feeling more positive this week.
"The reality is, we're starting to get tight in terms of the times," McDonald said.
"We're still optimistic, hopeful, but this time next week, I think we'll be in a position to get a better gauge on where he's at. We've still got a little bit to go.
"One of the benefits with Patty is that he has had the ability to prepare for test matches off shortened preparations.
"So if it was to be shrunken down, we'd be very confident that he would still be able to perform in the first test."
McDonald said Cummins probably needed to be back bowling a minimum of four weeks before the Perth test to have a chance to play without running the risk of further injury.
"Myself and Patty have spoken about that type of timeframe. Or else you start to bring in other risk factors – a) you're not skill ready, b) soft tissue injuries then become a real risk," he said.
"And if you were to do a soft tissue injury early in a series like the Ashes, then it's a long way back from where he's been. So, we'll be mindful of all those risk factors."
Losing Cummins for Perth alone would require a major shift in the Australian mindset, given the 32-year-old has been ever-present since making his Ashes debut in the 2017-18 series.
With Cummins fit and firing, Australia have never lost the urn since reclaiming it in 2017.
If he is not fit to play in Perth, former skipper Steve Smith is likely to captain the team with Scott Boland replacing him in the pace attack.
Australia also face England in tests in Brisbane, Adelaide and Melbourne before the series concludes in Sydney in early January.