Geelong star Mitch Duncan has been subbed out of his side's clash with the Suns due to a head knock and suspected concussion.

Duncan was tackled to the ground during the second-term of the match and came off the field shortly after looking groggy.

The Cats made the decision at half-time to end Duncan's day, with defender Zach Guthrie coming into the game.

Fellow Geelong midfielder Cam Guthrie also suffered a setback after suffering a shoulder knock but has been able to continue in the match.

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