Post-game scans revealed the injury after a knock Steven picked up late in the match, and the 27-year-old went straight to hospital and remained there overnight for observation.
Steven had 20 disposals and laid seven tackles during the 19-point loss.
“Jack sustained a punctured lung late in Saturday night’s game and was taken to hospital for observation,” GM of Football Performance Jamie Cox told saints.com.au.
“Due to the nature of the injury, Jack is unable to fly for the next day or two and will remain in Perth with club doctor Tim Barbour before flying back as soon as he is medically cleared.”
Steven has already been ruled out of St Kilda’s round three clash with Brisbane next Sunday at Etihad Stadium.