Jack Ziebell will head to hospital for scans after receiving a heavy knock to the chest in the first quarter of North Melbourne's clash with Port Adelaide.
The Kangaroos midfielder was helped from the field following a big collision midway through the opening term, and immediately went into the changerooms with club medicos.
The 24-year-old was substituted out of the game and will have precautionary scans, which a club spokesperson said was 'routine for this type of injury'.
PRESS PLAY above to watch the Ziebell incident | YouTube
It was a significant blow for the Roos, who already lost star midfielder Daniel Wells as a late withdrawal after failing to prove his fitness after struggling with an Achilles injury.
The club was also without Nick Dal Santo, who had hamstring surgery this week in what will likely keep him out for 10-12 weeks.
Ziebell was replaced in the Roos' line-up by substitute Ben Jacobs in what was a busy first term with injuries.
Kangaroos half-back Sam Wright was on the end of a bump which saw him head to the rooms, but he returned to the club's huddle at quarter-time.