The 23-year-old has a fracture in his right sesamoid bone and is expected to miss between 8-10 weeks.
While the injury isn't stress related and is the result of a training incident on Thursday, Steven will be unable to train for at least a month.
However, he does not require surgery.
The setback means the Saints will enter their round one clash with Melbourne without their two best midfielders, with Leigh Montagna to miss with suspension.
In a club statement, St Kilda's head of football Chris Pelchen said: "Jack has been training strongly throughout the pre-season but pulled-up sore after training today (Thursday).
"Scans conducted late this afternoon revealed a fracture in his right foot which is obviously disappointing for both Jack and the club on the eve of the 2014 season.
"It's important to note that the fracture is not stress related and followed an innocuous incident during training."
Sam Gilbert also looks to be an unlikely starter with the backman still at least a month away after surgery to repair a foot stress fracture in early February.
Gilbert is no longer wearing the moon boot and is straight-line running but still some time away from full football training.
The Saints could also look to elevate a rookie in the coming weeks if they decide to move untried ruckman Lewis Pierce - who is battling a back stress fracture - to the long-term injury list.
The Saints open their season against the Demons on March 22 at Etihad Stadium.
