On a humbling Old Boys' Day in Newcastle, the Knights blew an 18-6 lead early to leave themselves potentially needing to beat the Warriors next week to stay sixth.
Making matters worse is Young being at risk of missing the start of the finals, over a hip-drop tackle that left Sea Eagles star Tolu Koula with a syndesmosis injury.
Koula will require scans, but a grade-three charge would be enough to rule the star Newcastle winger out of the opening week of finals given they have a last-round bye.
"Every player is important. It was unfortunate," Knights coach Justin Holbrook said.
"When you see him chasing (Koula) down trying to stop a try, that's what he's trying to do. That's the way I looked at it and didn't realise he landed like that.
"He had both his feet on the ground, so hopefully all that helps him. It's out of my hands now, but we'll deal with whatever happens. Hopefully he's OK."
Saturday's loss ends whatever top-four hopes Newcastle had, while Holbrook's men must now beat the high-flying Warriors next week to guarantee a home final.
The result also kept Manly's faint finals hopes alive, as Kieran Foran's men snapped a six-game losing streak and showed off the attacking strike that had them firing mid-season.
Albeit remote, the Sea Eagles need to beat St George Illawarra and the Warriors, while also relying on South Sydney or North Queensland going winless for the rest of the year.
"I do like my maths and I've been crunching a few numbers," captain Tom Trbojevic quipped.
"Unfortunately we have put ourselves in a position where we will be relying on certain things to go our way.
"All we could focus on today was coming here and winning. We'll go back home and look to do that again."
Manly will be left wondering where this kind of attacking strike has been for the past two months, after sliding from third to only a mathematical chance of playing finals.
Centre Josh Feledy scored a hat-trick while filling in for Koula at centre, while Clayton Faulalo impressed in his first game in the No.6 jersey.
Trbojevic also looked dangerous on return, despite being felled a number of times during the match and at one stage being victim to a Tyson Frizell hip-drop tackle.
The Sea Eagles went after Newcastle's right edge at will, opening the scoring in the third minute when Lehi Hopoate set Faulalo free.
Manly were then vulnerable themselves down that side while Faulalo was off for a head injury assessment, with Kalyn Ponga, Fletcher Sharpe and Dom Young wreaking havoc as the Knights went 16-6 up.
But the game swung dramatically when Newcastle took a penalty goal to lead 18-6 and Jermaine McEwan coughed up possession on the next set.
From there, Trbojevic sent Caleb Navale over for a try on his return from an ACL rupture, and Feledy scored twice in the space of four minutes with Young in the bin to make it 24-18.
The Sea Eagles ran away with the game in the second half, again attacking the Knights edges in a sobering reminder of the challenge that lies ahead.
Dane Gagai should return from a hamstring injury for the finals, but any ban for Young will cause more changes on their right side.
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