The Swans have made a hot start to the season, unbeaten after big wins over Carlton and reigning premiers Brisbane.
But injuries to Gulden and fellow star Isaac Heeney (hamstring) in the final quarter on Saturday night soured their 44-point thumping of the Lions.
On Monday, the Swans issued a brief statement confirming two-time All-Australian midfielder Gulden will undergo surgery later this week.
"He is expected to be unavailable for approximately four months of football," the Swans' statement said.
A return in late July would leave Gulden with about one month to prepare for a finals series, if Sydney are still in the mix.
The 23-year-old was left shattered on Saturday night when he dislocated his right shoulder in a tackle from Brisbane's Lincoln McCarthy.
Lions forward McCarthy was making his AFL return after almost two years out with serious knee injuries.
Gulden was clearly in pain but was able to walk from the field and returned from the Swans' change room with his arm in a sling.
The latest injury setback comes after Gulden missed the first half of the 2025 season after injuring an ankle during the pre-season.
He returned to play 10 games, of which Sydney won seven.
Gulden made a strong start this season, gathering 27 disposals in the win over Carlton before tallying 20 touches and one goal against Brisbane.
Heeney also appears set for a stint on the sidelines after an early finish to the Lions clash with what the Swans called a "tight" hamstring.
"It was something that we don't want to risk at all," coach Dean Cox said post-match.
Sydney face another big test when they take on Hawthorn at the MCG on Thursday night.
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