Shaun Higgins' three-month absence has given North Melbourne their first significant blow after a perfect start to the AFL season.
The star forward hurt his right knee in Sunday's tight win over St Kilda and the Kangaroos were initially hopeful it was not a serious injury.
But Higgins needed surgery on Wednesday for a meniscus tear, putting him out of action until close to the finals.
North, who are the only unbeaten team, forecast he would be back around round 20.
The forward also needed minor knee surgery in the pre-season, but had played every game until this setback.
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"It's extremely unfortunate, but our focus is to ensure Shaun will have a thorough rehabilitation, which will allow him to return to the side late in the season," said North football manager Geoff Walsh.
"He's a tremendous character and an utter professional, so we know he will bounce back strongly from this setback."
The injury happened when St Kilda defender Sean Dempster fell across Higgins' right leg as the Kangaroos' star snapped a goal in the third quarter.
Higgins looked in trouble and needed to be helped from the field, but his knee was strapped and he played out the match.
The 28-year-old has been a great recruit for North since joining them in late 2014 as a restricted free agent.
He finished third in last year's club best-and-fairest voting and was an initial nomination for the All-Australian team.
The forward, who can pinch-hit through the midfield, has played 160 games - 31 for North and 129 with the Western Bulldogs.
