In an interview with sydneyswans.com.au, the hard-working midfielder said he was bedridden for almost eight weeks, missing a major component of the club's pre-season program.
The sickness, which Jack admitted may have partially been a result of celebrating "a little bit too hard" after the 2012 Grand Final win, planted serious doubts in the 26-year-old's mind.
"I missed a good six to eight weeks of training in January, during which I was pretty much bedridden.
"Certainly if you speak to any footballer they talk about pre-seasons and how important they are and they set you up for a big year.
"I was really questioning at the start of the year whether my body would be up to it."
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Despite the significantly limited preparation, Jack got himself fit for round one, peeling off 25 disposals and booting three goals against Greater Western Sydney.
He went on to play every game, finishing eighth in the Brownlow Medal, second in the Swans' best and fairest, and earning All Australian selection for the first time.
"To come out the other side and to have the year that I've had, I'm certainly rapt," Jack said.
The illness was first revealed by Swans coach John Longmire at the club's best and fairest dinner on October 4.
Addressing the audience as he presented the runner-up trophy, Longmire said Jack's ability to produce after such a difficult summer was a show of his strong character.
"For this bloke to actually miss a fair chunk of his pre-season and to come back in and have the season he had is an unbelievable achievement," Longmire said.
"It's a great example of how mentally strong he is, and how he doesn't use any excuse at all to get the most out of himself as a player."
Despite the significantly limited preparation, Jack got himself fit for round one, peeling off 25 disposals and booting three goals against Greater Western Sydney.
He went on to play every game, finishing eighth in the Brownlow Medal, second in the Swans' best and fairest, and earning All Australian selection for the first time.
"To come out the other side and to have the year that I've had, I'm certainly rapt," Jack said.
The illness was first revealed by Swans coach John Longmire at the club's best and fairest dinner on October 4.
Addressing the audience as he presented the runner-up trophy, Longmire said Jack's ability to produce after such a difficult summer was a show of his strong character.
"For this bloke to actually miss a fair chunk of his pre-season and to come back in and have the season he had is an unbelievable achievement," Longmire said.
"It's a great example of how mentally strong he is, and how he doesn't use any excuse at all to get the most out of himself as a player."
