The Bulldogs, with Bontempelli outstanding in his first game of the season, controlled proceedings after quarter-time in a convincing 18.19 (127) to 8.8 (56) victory at Marvel Stadium on Sunday night.
Bontempelli tallied 30 disposals, eight clearances and two goals to help the Dogs to the edge of the top eight with a 3-3 record - level with the Saints.
But the win has likely come at a massive cost after budding superstar Darcy's awkward landing in a first-quarter marking contest.
The 21-year-old key forward hyperextended his left leg in the incident and was substituted out of the match within minutes.
"The medical staff are pretty concerned at this stage and they're fearing the worst," Bulldogs football manager Matt Egan told the Seven Network at halftime.
"It looks like a possible ACL but we're just going to have to wait until tomorrow to get scans to confirm.
"At this stage we're fearing the worst, which is unfortunate because he's an incredible person and we'll obviously miss him.
"But until we get scans it's pretty hard to say too much."
It is just less than 20 years to the day since Darcy's father - 226-game Bulldogs great Luke - suffered the first of two serious knee injuries that sidelined him for most of 2005-06.
Broadcast vision in the second quarter showed Darcy's upset family, including his father, in the dressing room.
The 205cm forward had changed out of his playing kit when he returned to the Bulldogs' bench before halftime.
Bontempelli, who missed the first five rounds with a calf injury, started at half-back but quickly shifted into his customary midfield role as St Kilda got an early jump with four of the first five goals.
The Bulldogs wrested momentum and trailed by two points at quarter-time, having dominated clearances (12-6) and inside-50s (15-7) to that point.
It began to tell after the first break as Bontempelli kicked two of five unanswered Dogs goals in a one-sided second term.
The margin blew out in the second half and Tom Liberatore (28 disposals) provided a late highlight with a stunning rundown tackle on Saints speedster Brad Hill.
Ryley Sanders (32 disposals) and Bailey Williams (32) were busy, while Aaron Naughton and Ryley West (three goals each) took chances in attack.
Matt Kennedy also impressed with 22 disposals, 10 clearances and two goals.
Jack Macrae was one of St Kilda's better performers against his former side with 19 disposals and seven clearances.
Mitch Owens and Jack Higgins (three each) kicked multiple goals for the Saints.
But St Kilda collectively managed just four majors after their 15-minute burst at the start of the match.
