REJUVENATED Melbourne forward Tom McDonald is certain to start in attack on Saturday night against Sydney, despite the absence of injured key defender Adam Tomlinson.

Coach Simon Goodwin confirmed 21-year-old Harrison Petty will come in for Tomlinson, who tore his ACL against North Melbourne last week and will miss the rest of the season.

Petty has played 12 games over four years on the Dees' list – with 10 matches coming in 2019 – but looks set for an extended run at the top level after a groin injury ruined his 2020.

McDonald was swung into defence midway through the match against North to fill the hole, and has a history of playing at both ends of the ground.

The move means Sam Weideman looks likely to spend another week playing for Casey, having kicked 14 goals from his three official VFL matches.

Weideman started the season behind the eight-ball, recovering from stress fractures in his leg.

"[McDonald's] going to start as a forward, he's been one of our best forwards all year, so we'll get him back in that position," Goodwin said.

"He did a great job last week under difficult circumstances, but he'll be back in the forward line where he's played some strong footy.

"Harrison Petty will come into the team. Harrison's ready to play some footy, he's worked incredibly hard and is another highly valued young player that's put 3-4 years of development into his body and he's ready to play a significant role for us.

"It's come under really unfortunate circumstances with 'Tommo' going down, but he's been ready for a little while."

The undefeated, ladder-leading Demons are set to take on a young Swans side fresh off a nail-biting win over Geelong.

"We're really conscious of making sure to keep good cohesion with our team. Last week was the first time we had to make a number of changes leading in, with Viney (toe) and also with 'Tommo' going down in game.

"We'll look to keep our cohesion. We've still got some things to finalise, but the core nucleus of the squad that played last week will be playing again."

Goodwin said he was confident Weideman, who is out of contract at the end of the season, would remain at Melbourne despite his ongoing fight to earn a senior recall. 

"Yeah I am. Sam's a terrific young kid, a young player who has invested a lot into our footy club. I'm sure that'll sort itself out in time. He's a highly valued person and he's going to have a great career with the Melbourne footy club," he said. 

While Weideman appears unlikely to play seniors this week, Goodwin said the 23-year-old remained an integral part of the Demons' plans in 2021. 

"Sam's in a great spot. He's just got back from injury, he's played four games now (including one practice) back from an eight-week layoff, and his game is starting to come into great shape. There's no question he's going to push for selection pretty hard and be an integral part of our team at some point," Goodwin said.