Simpson wrenched his left knee when competing at the opening bounce of the second quarter. He collapsed to the turf in agony and was carried from the field on a stretcher.
The club's medical staff are confident that he has not ruptured his anterior or posterior cruciate ligaments, but they expect scans to reveal a medial ligament strain.
"The bad news is there appears to be something significant there," Geelong coach Chris Scott said following the big win over the Saints.
"They won't know the extent of that until midweek next week.
"It's really hard to speculate at this point. I guess we can confirm that it's not an ACL and it's not a PCL either.
"But they're still hopeful that Dawse can play a part in the rest of the season."
The injury is a cruel blow for Simpson, who was drafted in 2007 but played only five games in his first five years at Geelong.
After missing most of the lead-up to this season due to back surgery, he fought his way back and was called into the Cats' line-up for their round 13 clash with the Brisbane Lions.
He performed strongly at the Gabba and the following week at home against Fremantle, and the clash with St Kilda was his fifth consecutive match, meaning he had doubled his games tally in the space of five weeks.
But he now looks set to watch Geelong's tilt at another flag from the sidelines.
With Simpson injured and Mark Blicavs resting, injury-prone big man Nathan Vardy was forced to shoulder most of the rucking load against St Kilda.
And Scott was "really proud" of the way Vardy, who was playing his first AFL game since round 11, accepted the challenge.
"He's had a tough run, Vards," Scott said. "We've got a lot of optimism around the sort of footy he can play longer-term, but recently he's been dogged by injury a little bit and hasn't had a full run at it.
"And when Dawson went down we were a little bit hesitant to throw too much Vards' way, given his limited preparation. And I probably think he's a bit more of a forward than he is a ruckman.
"But he showed us tonight that he can shoulder that number one ruckman load for at least three quarters, so it's a real positive."
Geelong half-forward Travis Varcoe was also full of praise for Vardy's effort.
"You could just see it in his eyes; he just knew that he had a fair weight on his shoulders," Varcoe said. "That's what Geelong players are about.
"From a teammate's point of view, I thought he was our best."
All of Geelong's big men were impressive contributors. Tom Hawkins booted five goals, while James Podsiadly took a contender for mark of the year.
However, Podsiadly is set to attract the scrutiny of the Match Review Panel for the second straight week after collecting St Kilda midfielder Jack Steven with a forearm to the head when he tried to fend him off a few minutes prior to half-time.
Perhaps the greatest positive to come out of the evening was the fundraising effort to benefit former Cats VFL player Casey Tutungi, who was recently diagnosed as a quadriplegic after an accident in a local football match.
A number of injured Geelong players, including Paul Chapman and Allen Christensen, did their bit by rattling tins outside Simonds Stadium in the lead-up to the clash with the Saints.
"On behalf of the Geelong footy club I would like to thank everyone in the wider Geelong community for their response to the Casey Tutungi game," Scott said.
"It appears that across all the local leagues in Geelong we've raised $80,000 today, which is great for Casey and his family.
"But we also understand this isn't going to be a short-lived thing, so the Geelong footy club will be in for the long haul."
