Petty, a premiership-winning tall defender, has been ruled out for at least a month with a foot injury after impressing in his new role in attack this season.
He kicked two goals in Saturday's big win over Hawthorn before being substituted out at half-time.
The injury setback opens the door for regular forwards Brown and McDonald, who have played just three and four senior games respectively this season.
Brown kicked nine goals in the opening three weeks before he was briefly sidelined by a back injury and lost his spot in the side.
McDonald has not played since a goalless, six-disposal effort in the round-five loss to Essendon, which followed a four-goal haul against West Coast a week earlier.
"One of those guys will probably get their chance this week," Demons coach Simon Goodwin told Fox Footy.
"Part of this season is about exploring.
"We want to continue to be a team that gets better through the year and one of our explorations was around Harrison and his ability to play forward.
"We've liked what we've seen so far, so we would've liked to have seen that a bit more, but we get a look at a different mix this week."
Young forward Jacob van Rooyen is likely to hold his spot in Melbourne's side, with ruck duo Max Gawn and Brodie Grundy also presenting marking targets in attack.
While Petty has been sidelined, the Demons breathed a huge sigh of relief when scans cleared star midfielder Christian Petracca of serious ankle damage after the win over Hawthorn.
"You get concerned when one of your better players is down but he's fine," Goodwin said.
"He ran today, he'll fully train (on Tuesday) and he'll be fully available."
17°C