Windhager, who start on the ball at RSEA Park for his side's second and final practice match, was pictured on the interchange with his arm/hand covered in ice.
It is suspected that Windhager has suffered a hand injury, with the teenager forced out of competing in the remaining quarters for Friday's match.
The setback could place Windhager in doubt to commence his second season as a Saint on time, with the club to be sweating on their young bull.
The injury comes after amid a frustrating pre-season for senior coach Ross Lyon and his men, who have been dealt injury blow after injury blow as Round 1 nears.
The Saints are already without key forward pair Max King (shoulder) and Jack Hayes (foot) and midfield duo Jack Billings (leg) and Zak Jones (Achilles) for their season opener, while senior players in Tim Membrey (knee) and Seb Ross (calf) are in a race against the clock to be fit to face Fremantle later this month.
The Saints were also without veteran Brad Hill for Friday's matchup with the Bombers due to personal reasons.
Caldwell was assisted off the field by medical staff and was assessed on the sidelines. He suffered an ankle sprain, with the young midfielder ruled out of the final term.