Leach's lower-back stress fracture, diagnosed last Sunday, prompted England to ask Moeen if he would consider reversing his retirement from Test cricket. He agreed, and will link up with the rest of their squad for the first two Ashes Tests during Tuesday's training session at Edgbaston, with the first Test starting on Friday.
Since Stokes took over as captain a year ago, Leach has bowled 515.1 overs in Test cricket - 179 more than any other England bowler. And with Moeen short on red-ball practice, McDonald believes that England will have to "operate differently" in the Ashes.
"I think there's a challenge there, isn't there?" McDonald said, speaking at The Oval after Australia won the World Test Championship final. "Mo hasn't played a red-ball game for an extended period of time. No doubt he'll be prepared - he's had a couple of weeks' notice.
"But I think Jack Leach really complemented that attack and his wicket-taking ability and the way Ben Stokes has used him has been a little more aggressive and has reaped some great rewards.
"So, yeah, it will look different with Mo there. Mo has 190-odd Test wickets as well [and] strengthens their lower order, so it won't be the same but they'll have to operate fractionally differently - no doubt about that."
Moeen has not bowled more than 10 overs in a match since his most recent Test, which came against India at The Oval in September 2021. Michael Vaughan, the former England captain, suggested in his Telegraph column that he might be employed as "a match-up bowler" against Australia's left-handers.
"His bowling? Well, it will not be consistent," Vaughan wrote. "But he has the ability to bowl truly great deliveries and, if he could find 12 of those in the series to bring wickets, England will be happy.
"Australia have a heap of lefties for him to bowl at, right down to Mitchell Starc and Josh Hazlewood in the tail. It will be all about key wickets at key times, probably with the sort of aggressive fields Leach has been given.
"For a while I have thought England are a bit like a T20 set-up. Moeen will be used at certain moments, possibly even as a match-up bowler."