Ridley was substituted out after a nasty knock to the hip before the Bombers capitulated during the third quarter of the 17.6 (108) to 8.7 (55) loss at Marvel Stadium on Saturday.
The Saints piled on seven goals to one during the decisive period, including three majors to skipper Jack Steele, as Essendon failed to fire a shot in one of the most important moments of their season so far.
It led to a third consecutive defeat for the Bombers and sixth in their past eight games, leaving them in a precarious position in a hotly contested race for finals spots.
Sitting third at the end of the bye rounds, Essendon could finish this weekend out of the top eight with four home-and-away matches left to play.
Essendon coach Brad Scott says his side's lack of effort was "extremely disappointing". (Rob Prezioso/AAP PHOTOS)
Bombers coach Scott had no explanation for his side's "lack of competitiveness" and "negligible pressure" in such an important fixture.
"It's extremely disappointing," Scott said.
"The end result is disappointing but more disappointing than that was our lack of effort and intent, which has been present for most of the year.
"Particularly in the third quarter, our effort and intent in the contest was really poor.
"Across the course of the game it was poor and it doesn't take much, if you drop off in that area, for you to look second-rate.
"We looked third-rate today."
Ridley missed the first half of the season with a quadriceps injury and took a heavy blow to the same leg in a marking contest.
"It was to the front of his hip and we had some concerns around his quad that he's injured before," Scott said.
Joining the party 🔨 — St Kilda FC (@stkildafc) pic.twitter.com/gvQ931jlGgJuly 27, 2024
"He had a fair bit of pain and couldn't function, so we had no choice but to sub him off.
"We'll scan that and fingers crossed.
"It presents like a knee to the hip which is a pretty severe cork and hopefully it's nothing more serious."
Nasiah Wanganeen-Milera starred for St Kilda, who are out of finals contention but have won three of their past four games in a late-season purple patch.
Dashing playmaker Wanganeen-Milera had 30 disposals and six score involvements at half-back, and capped off a fine display with his first goal of the season.
Cooper Sharman kicked three goals and had 10 marks in a career-best performance, while Jack Sinclair (28 disposals), Riley Bonner (22) and veteran Zak Jones (22) were all busy.
Skipper Steele (21 touches) and ruckman Rowan Marshall (27 disposals, eight clearances) led from the front.
Anthony Caminiti (three) and Tim Membrey (two) also kicked multiple goals as St Kilda racked up their second-highest score of the season.
Anthony Caminiti booted three goals as St Kilda scored a 53-point win over Essendon. (Rob Prezioso/AAP PHOTOS)
The Saints' conversion rate - 23 scores from 43 forward entries - and accuracy were superb, highlighted by Owens, Membrey and Caminiti all nailing goals from deep in the same pocket.
In a telling statistic, the Saints finished with a massive tally of 133 uncontested marks while winning the contested mark count 16-5.
"I thought we played really smart football," St Kilda coach Ross Lyon said.
"Our opposition (analyst) understands them really well and we executed on it.
"Everyone stood up but some of our younger guys like Wanganeen-Milera and Sharman and (Ryan) Byrnes ... there was a bit to like."
Essendon captain Zach Merrett (24 disposals, one goal) never stopped trying in a well-beaten side, while Dylan Shiel kicked two goals.
Bombers midfielder Darcy Parish had just 13 disposals on his long-awaited return from injury.
