Young pushed off with no-one around him in the 55th minute and clutched his lower leg before being assisted from the field.
The up to eight-month recovery ahead of the NSW second-rower for the rupture will also rule Young out of the World Cup for Australia.
Young's teammates hung on despite a Rabbitohs rally at GIO Stadium on Saturday as the late run of last year's minor premiers at an unlikely finals berth went into overdrive with a third consecutive victory.
Coach Ricky Stuart, who confirmed the rupture, said Young's misfortune was a massive blow.
"I feel for him personally," Stuart said.
"He is such a massive part of our team.
"It hasn't hit him yet, but he misses the rest of the season and misses the World Cup. I've been there personally and it hits you in the next couple of days, but he's got the right people around him."
The Raiders' spine was in scintillating form in a five-try first-half blitz to lead 28-6 at the break.
Fullback Kaeo Weekes, halves Ethan Strange and Ethan Sanders and dynamo hooker Owen Pattie were too quick between the ears and across the ground for the Rabbitohs.
It was the most first-half points by the Raiders against the Rabbitohs since 1996.
After their round 16 loss to Melbourne, the Raiders were in 16th place and eight points outside the top eight.
Now, whatever the results this round, they will finish round 20 on 20 points and just four points behind the eighth-placed team and still in with a shot at playing finals.
Strange is an electrifying ball runner and it was his giant left foot step on Rabbitohs half Ashton Ward, followed by a typically robust tackle bust through David Fifita, that set up Pattie under the sticks.
The Raiders were in true Green Machine mode for their second try, which went through a dazzling 11 sets of hands. Young finished it with a freakish pick-up low down.
Ward, who recently inked a two-year extension, scored an individual try with a show-and-go close to the line, before a Strange intercept and burst was brilliantly finished by Weekes in a 90m play.
Pattie's speed out of dummy-half was the catalyst for barnstorming second-rower Noah Martin to crash over. Sanders landed a 40/20 and Weekes added a second.
The Rabbitohs, who remain in sixth position, started the second half with more intent, with talisman Cody Walker sending winger Alex Johnston in for his 20th try of the season.
Captain Cameron Murray steamed on to Ward's pass and the margin was just 10.
Raiders winger Jed Stuart swooped on a loose pass when the Rabbitohs looked like scoring and raced the length to score.
Walker scored his side's third try of the second half after he appeared to knock on in the lead-up, but the Raiders hung on to move to 11th on the ladder.
Rabbitohs coach Wayne Bennett was clearly unimpressed with his side's display.
"They played the second half the way they should have played in the first half," he said.
Bennett said it was inconsistency that concerned him the most.
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