Sydney could be without Isaac Heeney for the first few weeks of the season after it was announced on Tuesday that the star youngster has been diagnosed with glandular fever.
Heeney sat out the Swans' practice match against Greater Western Sydney on Friday night due to the virus which, at the time, was a mystery.
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Sydney football manager Tom Harley said the news was disappointing for both Heeney and the club.
"He has had a terrific pre-season to date," Harley said of Heeney.
"However, Isaac is receiving the care and rest he needs so we're confident he will return to play early in the season and I've no doubt he will make a solid contribution when he returns."
The Swans have a tricky start to the season with matches against reigning premiers Western Bulldogs (away), West Coast (away) and GWS in the first five rounds.
The 20-year-old had a great 2016 season. He averaged 17 disposals, and was ranked third at the Swans for goals (28), equal third for marks inside 50 and fourth for marks.
Since being taken at pick No.18 in the 2014 draft as an academy selection, Heeney has played 38 games, including last year's grand final loss to the Dogs, and kicked 44 goals.