SYDNEY key defender Tom McCartin's short-term future is unclear after he again entered concussion protocols following a knock to the head late in Sunday's narrow victory over St Kilda.

McCartin was felled as Saints debutant Max Heath reached to take a mark on the lead and hit the turf face first.

SAINTS v SWANS Full match coverage and stats

It sent concern around Marvel Stadium given the history of both McCartin and his brother Paddy with concussion.

Tom missed nine games in 2023 after complications following a concussion suffered in round four of that year, while he missed another four games last season due to a seperate concussion.

Brother Paddy retired from the AFL in 2023 at the age of 27 due to the impact of the multiple head knocks he suffered.

Given Tom's history, he was immediately subbed out of the game on Sunday and coach Dean Cox confirmed the 25-year-old has entered concussion protocols.

"We entered into protocols, that's the first thing," Cox said.

"You just ruled (him) out of the game with symptoms and entered straight into protocols. So, he's okay. Now we'll assess him in the next couple of days.

"We'll support him, but he was walking around in the change rooms, which is really good.

"Straight away, symptoms suggested that he'll go into protocols."

Sydney had already made its substitute a quarter prior – deciding to withdraw Aaron Francis from the game to get the fresh legs of Riley Bice onto the field – so was left with just three available players on the bench for the remainder of the game.

"That's a decision that we made," Cox said when asked whether the sub rule had disadvantaged his side in that instance.

"My ideal thing with the sub rule is, if you have five players there, why can't they play? Do you need to have a sub? My thing is, just play all five."

Ultimately the Swans won out thanks to a neat Errol Gulden goal in the final four minutes, after which it defended grimly to its five-point lead.

For Gulden, it was a moment of relief after a tough, contested day at the office.

"Probably more relief than anything, to be honest," Gulden told AFL.com.au of the goal.

"We didn't play our best today, clearly, and it probably wasn't my best either. So, to pop up with an important moment is always good. Most importantly, get the four points and improve and roll on."

It was a tap from in-form ruck Brodie Grundy that made it possible, with the tall crucial to the victory to continue his brilliant recent form.

"He's been unbelievable. Obviously, I've only been in for the four games, but I just noticed a difference that he's made for our midfield group in particular, and allowing us to probably get a little bit more active around stoppage and contest.

"And his ability to hit the ball 360 degrees makes it really hard for opposition players to defence, and it makes it good for us getting on the end of it."

Another Swan who stepped up late in the game to push his side over the line was Will Hayward, who had five disposals, three marks and two goals in the final quarter, including a pivotal grab in defence in the closing minutes.

"I think one thing you have to make sure (of) is you know how to close moments out. Will Hayward, when he drops behind the ball and takes a saving mark, not only what he was doing ahead of the ball, but to be able to do that behind the ball was really telling," Cox said.

For St Kilda coach Ross Lyon, there was a sense of frustration that his side let one slip. There were positives for Lyon given his young team showed a clear ability to take away Sydney's ball movement for much of the game, but going down by five points at the death left a sour taste in his mouth.

"We're frustrated, and angry, really. But it's like, it's okay to be angry. It's what you do with your anger. It's okay to feel that emotion, but channel it. Not into dysfunction, not a pity party. Channel it into improvement, and that's what we're doing," Lyon said.

He alluded to the youth and inexperience in the side, which included debutant Max Heath and third-gamer Alix Tauru, but that skill needs to come with the spirit and desire the side plays with.

Lyon also confirmed that small forward Dan Butler suffered a dislocated elbow in the second quarter, and is set for some time on the sidelines.

"Butler, dislocated elbow, or partial, they're a nasty injury, they decouple to the ligament. So, we'll have a scan. He's pretty tough, at least a month. Could be the season," Lyon said.