Hawthorn has conceded Tom Mitchell will "most likely" miss the entire 2019 season.
The club confirmed Mitchell had broken his leg at training on Friday morning.
The 25-year-old will undergo surgery over the weekend.
Hawthorn's general manager of football Graham Wright said the injury was devastating for the club.
“For Tom to suffer an injury like this is incredibly disappointing for both him and our footy club,” he 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.”
Mitchell has starred in his two seasons at Hawthorn, winning the 2018 Brownlow Medal with 28 votes.
He averaged 35 disposals, eight clearances and six tackles a game last year.
It was a massive year for the Saturday Rub in 2018...until Billy's quiz rolled around!