Patton screamed in pain when he was tackled into the turf by Saint Rhys Stanley midway through the second term at Manuka Oval.
The 2011 No.1 draft pick immediately clutched his right knee and was unable to stand up.
The 19-year-old was helped off the field and down the race by two GWS officials. Giants coach Kevin Sheedy later confirmed Patton had torn the anterior cruciate ligament in his knee and would miss the rest of the year.
Five talking points: Greater Western Sydney v St Kilda
It was a sour point for the record crowd of 11,092 - GWS's best at Manuka - in a game the hosts lost on the back of a dreadful opening term.
St Kilda became just the third team to hold the Giants scoreless for a whole quarter, easing to a 34-point lead at the first break en route to a 21.14 (140) to 10.8 (68) victory.
The Saints had booted 5.4 to 0.0 at quarter-time. If they kicked straight, the game would have been as good as over then.
As it was, the visitors' advantage was never called into question.
They were harder at the ball, as depicted by a 139-109 contested possession count, while on-ballers David Armitage, Nick Dal Santo and Leigh Montagna used it intelligently.
Tom Scully tries to wrap up Nick Dal Santo in a tackle in Canberra. Picture: AFL Media
This was also a game of contrasting forward lines - one side had a bounty of targets while the other struggled to find one.
ForGWS, ruckmen Dean Brogan and Jonathan Giles combined for four goals,while midfielders Devon Smith and Adam Treloar also kicked two apiece.
Incontrast, St Kilda small forwards Stephen Milne (three goals), AhmedSaad (three) and Terry Milera (two) were magnificent in both theirpressure and presence up forward.
Beau Maister (2.3) and captain Nick Riewoldt (3.2) were also ominous, albeit not as accurate as the side's mosquito fleet.
Armitage kicked four goals and tallied 31 disposals to be best afield.
Aserious setback would be shattering for Patton, who had started to showsigns of his potential after benefiting from a full pre-season.
Thetalented teenager, who attracted comparisons to Jonathan Brown in hisjunior career, had his maiden season curtailed by surgery on his leftknee in Sweden last year.
St Kilda had its own injury concern, with Jack Newnes (concussion) substituted out early in the second quarter.
Click here to vote for the best three GWS players from the game
Click here to vote for the best three St Kilda players from the game
ST KILDA 5.4 10.8 15.12 21.14 (140)
GREATER WESTERN SYDNEY 0.0 5.1 8.3 10.8 (68)
GOALS
St Kilda: Armitage 4, Saad 3, Milne 3, Riewoldt 3, Milera 2, Maister 2, Steven, Lee, Montagna, Gilbert
Greater Western Sydney: Treloar 2, Brogan 2, Smith 2, Giles 2, Patton, Cameron
BEST
St Kilda: Armitage, Steven, Montagna, McEvoy, Roberton, Milera
Greater Western Sydney: Smith, Ward, Treloar, Bugg, Giles, Kennedy
INJURIES
St Kilda: Newnes (concussion)
Greater Western Sydney: Patton (knee)
SUBSTITUTES
St Kilda: Farren Ray replaced Jack Newnes in the second quarter.
Greater Western Sydney: Sam Reid replaced Jonathon Patton at half-time.
REPORTS
Greater Western Sydney: Devon Smith reported in the fourth quarter for charging Jarryn Geary
St Kilda: None
Umpires: Fila, Burgess, Chamberlain
Official crowd: 11,092 at Manuka Oval
