MELBOURNE has confirmed young midfielder Maddy Guerin ruptured her left ACL in Friday night's loss to St Kilda, the team's fourth such knee injury since late December.

It also pushes the team's injury and unavailable list to eight of 30 players, paving the way for a possible surprise return to AFLW for ex-Adelaide premiership forward Sarah Perkins.

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Guerin joins Ainslie Kemp, Shae Sloane and Katherine Smith as Demons with torn ACLs, with Lauren Pearce (knee), Lily Mithen (ankle), Jackie Parry (hamstring) and Bianca Jakobsson also sidelined.

In better news for the club, Sinead Goldrick (concussion), Shelley Heath (finger) and Aliesha Newman (foot) are expected to be available for Friday night's match against Collingwood after picking up injuries in round three.

If clubs have fewer than 23 players available, they can employ the services of train-on players as emergencies, and as on-field players if there are fewer than 21 fit players.

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Perkins – who was delisted at the end of the 2018 season – and veteran defender Kirby Bentley (formerly of Fremantle and Carlton) are the club's elevated train-on players.

Guerin, 20, had suddenly dropped to the ground in the second quarter when attempting to change direction without any contact.

The club initially believed it was a dislocated kneecap, before scans confirmed the extent of the injury.

"Unfortunately, today's (Sunday's) scans confirmed that Maddy suffered an ACL injury as she attempted to change direction during Friday night's game," AFLW football manager Daniel McPherson told the Melbourne website.

"Maddy is a highly valued member of the team who works incredibly hard. We'll be ensuring she has the support she needs to make a full recovery."