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The forward who kicked a match-winning four goals in last year's AFL grand final has "low-grade" medial-ligament damage following last week's win over the Western Bulldogs in Launceston.
"What that effectively means is he'll be out for somewhere between two and three weeks," Hawthorn football manager Chris Fagan said.
Ladder leaders Hawthorn might not regain Gunston until their last-round clash with Collingwood on August 29, which could hamper Hawthorn's bid to finish in the top two and avoid a potential interstate trip during the finals.
Gunston has kicked 44 goals in 2014, and while team-mates Jarryd Roughead (50) and Luke Breust (46) have booted more, the former Adelaide sharp shooter is considered a finals weapon for the Hawks.
Meanwhile Sam Mitchell (flu) and Isaac Smith (calf tightness) are set to return for Saturday's game against Melbourne.
But Norm Smith Medallist Brian Lake is not eligible to return from a four-game ban until Round 21 against Fremantle on August 17 in Perth.
Brad Sewell will also be available to return this weekend after recovering from a hamstring injury sustained in Round 16.
But the reigning premiers might not want to rush the dual premiership midfielder's return given a further setback could end his season.
Key Hawthorn forward to miss three weeks
