The 23 year-old suffered the injury during Sunday's win against Hawthorn and is set to undergo surgery on Monday afternoon.
Dawes was one of the Magpies' best performers in the round two thrashing of the Kangaroos with 26 possessions and two goals, and his exit may affect not only Collingwood's selection dilemmas.
Brad Scott added extra ammunition in defence against the Saints, but may opt for more pace this week against the hard-running reigning premiers.
Kieran Harper, Shaun Atley and Matt Campbell could be in the mix if that's the case, while Robbie Tarrant made another push for selection by keeping Brendan Fevola to two goals in the VFL.
Big man Hamish McIntosh is also edging closer to a return, and may be the perfect foil for in the in-form Todd Goldstein against Darren Jolly and Leigh Brown in the ruck.
