Experienced North Melbourne defender Daniel Pratt is expected to be out of AFL action for a month with a fractured thumb.

The backman sustained the injury shortly before halftime in Saturday's agonising one-point loss to Sydney at Etihad Stadium, but played out the game.

But he has since had surgery to have a plate and screw inserted.

The Roos have lost another defensive stalwart Michael Firrito to suspension.

With fellow defender Nathan Grima already out with a finger injury, it leaves the club's defence badly depleted for Sunday's clash with Adelaide.

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