He was only out by about 19 minutes, as the Swans had to come from being behind for the vast majority of the match to prevail in a pulsating duel.
Jai Serong's snapped goal with 14 seconds left marked the first time Sydney had been in front since midway through the first quarter on Sunday evening at the SCG.
They won the next centre clearance to overcome an outstanding challenge from the Saints, 15.14 (102) to 15.12 (102).
Sydney trailled by 33 points in the second term and were below their best for a large chunk of the game.
They also lost key defender Tom McCartin to another concussion, after he backed into a marking contest and Liam Ryan cannoned into him.
Justin McInerney also suffered a hamstring injury and sat out the second half.
"We're starting to build a lot of belief when we're in those positions, we can still get it done," Swans coach Cox said.
"Our side is starting to really mature in those moments, when it matters most."
Serong and Chad Warner led the way for Sydney, while Cox praised Isaac Heeney's last quarter as "phenomenal" after he'd absorbed close attention from Marcus Windhager.
Big-name St Kilda recruits Liam Ryan, who kicked five goals, Tom De Koning and Jack Silvagni all starred.
Ryan's fifth goal in the last quarter came after his contender for mark of the year, a soaring effort from the man known as Flyin' Ryan.
He also celebrated his fourth goal with an impressive Michael Jackson dance impersonation.
De Koning had probably his best game since joining the Saints and Silvagni looked to have taken the game-saving mark late in the last term, before Heeney's slick handball from a stoppage set up Serong for the match winner.Â
Saints coach Ross Lyon was flattened by the loss, saying his team had done a lot right for no reward.
"It's a game of centimetres, really, as opposed to inches. An inch is probably a bit long for what we experienced tonight," he said.
"The group's hurting, emotionally, but in the end you have to come up next week. That's the business.
"It can take the wind out of your sails, but you can't let it.The best individuals and the best groups push through these until they get to where they want to go. That's the challenge.
With Nasiah Wanganeen-Milera, Ryan and Mitch Owens back in the team, St Kilda were a completely different proposition in attack after going goalless in the first half against Hawthorn last round.
Lyon said the first half against Hawthorn had been "a little bit of a roadblock". Clearly, the Saints learned their lessons quickly.
They had done their homework, bringing excellent pressure to bear on the Swans for much of the game.
"They have a clear style of play and teams that have beaten them execute what we did. It's a pretty simple formula, it's out there in AFL footy," Lyon said of the second-placed Swans.
Lyon was rapt with De Koning and Rowan Marshall as his ruck tandem.
"It's a strong performance and that's going to continue to evolve. They've had a few of these and they've had a couple that have been quiet," he said.
"We're still looking to embed that and grow."
8°C