SYDNEY Swans defender Zak Jones faces a battle to be fit for this year's finals series with news that he'll miss at least a month of footy with an ankle injury.

Jones suffered a high ankle sprain on his left leg in the last quarter of the Swans' disappointing five-point loss to Hawthorn at the SCG on Thursday night, and has been ruled out for 4-6 weeks.

With just six home and away matches remaining, Jones could be right to play before the Swans' first final if there's no setbacks with his rehabilitation, but if the injury takes longer to respond, Swans' coach John Longmire could have a tough decision to make at the selection table.

The 21-year-old has played 14 games this year after overcoming off-season knee surgery, and had shown his best form over the past month as an attacking backman alongside fellow youngster Callum Mills and co-captain Jarrad McVeigh.

Jones had a career-best match against Melbourne and older brother Nathan in round 13 when he gathered 25 possessions and laid eight tackles in his side's 55-point win at the SCG.