HAWTHORN star Tom Mitchell's season is likely over after he broke his left leg at training on Friday morning.

The club later confirmed he suffered clean breaks in his tibia and fibula.

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Returning in the first session following the Christmas break, the Brownlow medallist sustained the injury during a tackling drill.

Hawks doctor Michael Makdissi told hawthornfc.com.au it was unclear at this stage what the exact timeline for Mitchell's rehab program would be.

"The X-rays confirmed two clean breaks to the tibia and fibula bones and Tom will undergo surgery this weekend to repair them," said Makdissi.

"The positive news for Tom is that there are no other associated injuries and as such it is just a matter of waiting for the bones to heal.

"Once this occurs, we will be in a better position to dictate the speed of Tom's rehabilitation and ultimately his return to the playing field."

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The 25-year-old will have surgery over the weekend, and then begin a rehabilitation program.

"For Tom to suffer an injury like this is incredibly disappointing for both him and our footy club," Hawthorn football manager Graham Wright said.

"He will undergo surgery over the weekend and as a result will most likely miss the 2019 season.

"However, Tom is a very resilient young man and we have no doubt that he will tackle his rehab with the same professionalism and diligence with which he approaches his training."

A new AFL rule allowing clubs to sign previously listed players as Category A rookies will give the Hawks until the eve of the 2019 season to replace Mitchell.

Previously, clubs couldn't replace players who had suffered season-ending injuries or who had unexpectedly retired after list lodgement deadlines had fallen.

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However, the AFL's recently introduced pre-season supplemental selection period (SSP) rules now enable clubs to replenish their lists with new signings through until March 15.

Mitchell averaged over 35 disposals last year and polled 28 votes to win the Brownlow Medal.

In an incredibly prolific 2018 season, Mitchell ranked No.1 in the competition for clearances and effective disposals per game, No.2 in contested possessions per game, No.8 in inside 50s per game and No.11 in tackles per game.

At the Hawks he had a whopping 90 more clearances than the club's next best - Jaeger O'Meara (102) - last year.