McInnes was hurt in the opening minutes of the Magpies' VFL clash with Essendon on Friday and looked distraught as he grabbed the joint.
Medical staff helped the 23-year-old to his feet before he limped off the field, with scans later confirming the damage.
It was McInnes' fourth game back in state league action after he missed most of last season with a similar injury to his right knee.
"Reef's injury is incredibly disappointing news, particularly given the commitment and hard work he has put in over the past 12 months to return from his right ACL," Collingwood football manager Charlie Gardiner said.
"He's an outstanding character and has shown enormous resilience and positivity to get back to playing.
"While this is undoubtedly a difficult setback, our entire program will be behind Reef every step of the way during his recovery."
A first-round draft pick in 2020, McInnes has played just 23 AFL games in six seasons with Collingwood.
In better news out of the VFL fixture at Olympic Park Oval, Magpies livewire Bobby Hill made a successful return after his time spent away from the club on personal leave.
The 26-year-old small forward started in the centre square and gathered the first possession of the game, but required treatment for a nasty cut below his left eye.
Hill, who won the 2023 Norm Smith Medal, finished with 11 disposals and one goal in Collingwood's 11-point win.
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