Off-season recruit Brodie Grundy will take over lead ruck duties against Sydney on Sunday as the Demons look to bounce back from a tough loss to Brisbane.
Helping their cause, Key defender Steven May will return from a calf injury to face Swans champion Lance Franklin, who is back after a one-match suspension.
The flow-on effect of Gawn's injury means further adjustments are required in attack for Melbourne.
Van Rooyen, a 193cm and 96kg key forward, was the club's first pick at the 2021 draft - No.19 overall - and has had to bide his time at VFL level.
"He's worked incredibly hard to develop his game over the last two years and we've been pretty hard on him about getting his game to the right level," Demons coach Simon Goodwin said.
"He's continued to execute that through pre-season and the first few weeks, so he'll debut for us and that's exciting.
"He's a young power forward coming into the game."
Van Rooyen, 19, will line up in attack alongside leading goal-kicker Ben Brown and fellow tall Tom McDonald in a bid to test the Swans' key defenders.
"He's trained with these guys for a few years now, so he's got pretty good cohesion with this group already, and one thing you'll see from big Rooey is his ability to compete aerially," Goodwin said.
"He goes at the ball hard, he creates a contest and his follow-up at ground level is exceptional.
"We won't be asking him to do too much other than that for us."
Goodwin backed former Collingwood star Grundy to step up in Gawn's absence, with the skipper sidelined for at least the next four weeks.
"This isn't abnormal for Brodie," Goodwin said.
"He's played as a sole ruckman for a big part of his career and he's done it incredibly well.
"We're excited to have Brodie have this opportunity.
"It's not what we were planning but it's certainly an opportunity that we think we can actually try to exploit."
May missed the opening two rounds of the season with a calf injury and will be welcomed back in defence.
"We've been cautious with Steve to make sure he's 100 per cent ready to go but we need his leadership and his ability to win one-on-one contests," Goodwin said.
"He's a really important player to us."
May restricted 1000-goal champion Franklin to just seven disposals - and held him goalless - when the teams met in last year's qualifying final.
But Sydney won the game and Goodwin conceded John Longmire's side has had the wood on the Demons in recent years.
"They have had some success against us and we're under no illusions that we need to make some changes to really take the game to them on both sides of the ball," Goodwin said.
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