St Kilda co-captain has avoided an Achilles injury, with scans confirming a high-grade calf injury.
Sinclair is in jeopardy of missing the remaining of the season but no timeline has been confirmed by the club.
Star recruit Tom De Koning will be further examined following two fractured ribs and a punctured lung, which is an injury he has sustained in the past, while fellow ruckman Rowan Marshall has been cleared of any serious shoulder injury.
“It's really disappointing for Jack and we're all behind him as he begins his recovery,” General Manager of Football Lenny Hayes said.
“Jack has done a terrific job since being elevated to co-captain this season and we know he'll continue to provide an important leadership and support role within our group off the field throughout his recovery.
“Despite Tom initially showing no signs of internal damage, our decision to send him to hospital was taken with every precaution. Tom's health and wellbeing remain our number one priority, and we will be right behind him as he navigates this challenging recovery.
“We're pleased that Rowan has been cleared from serious damage, with the bye coming at an opportune time for him to rest and recover.”
The Saints have the bye in Round 16.
