The 21-year-old was taking part in the Suns' match simulation on Thursday, but fell to the ground during a contest with No.5 draft pick Mac Andrew.
King was quickly supported by coach Stuart Dew and Suns teammates.
But the club believe King has likely suffered the dreaded season-ending injury.
King was taken for a scan but the Suns are not optimistic about his chances of avoiding surgery.
"Ben left the field with an injury and was assessed by club medical staff at the time," Suns football manager Wayne Campbell said.
"Ben will get scans this afternoon and we'll wait on the results of those before taking any further action."
King is out of contract at the end of the 2022 season, with clubs in Victoria already trying to prise him away from south-east Queensland.
He has kicked 89 goals - 47 of them coming last year - in 53 games since debuting in round nine, 2019.
King is a player the struggling Suns can't afford to lose with the club yet to play finals since entering the AFL in 2011.
Gold Coast finished 16th last season with seven wins.
King's twin brother Max plays for St Kilda while Essendon have also been heavily linked to Ben.
There has been speculation about Ben returning to Victoria ever since he was drafted by Gold Coast back in 2018.
The Suns were unimpressed by a cheeky tweet from St Kilda's social media team on draft night that read: "Ben, if you ever want to come home."
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