Named an emergency against Hawthorn, and indeed for all three of the Swans' finals matches last year, Armstrong came about as close as possible to a premiership-winning campaign without actually playing.
"After the boys ran out, I was down in the rooms with (fellow emergencies) Tommy Walsh and Trent Dennis-Lane," Armstrong told AFL.com.au.
"We just had a shower, got changed and sat with all the other players and family right behind the goals.
"It was kind of a strange thing really. You felt part of it, but then you didn't.
"It was really weird, but awesome at the same time, seeing some of my best mates win a flag."
Naturally thrilled for his teammates, that day steeled the running defender to do everything he could to be a regular member of the side.
It has been an encouraging start so far with Armstrong appearing in five of the team's opening six matches.
Saturday's clash with Hawthorn – their first meeting since the Grand Final – will be Armstrong's fifth consecutive game in the senior team.
He has partly benefited from an abdominal injury to fellow defender Rhyce Shaw, although both were selected in the same side for round one.
"Obviously when Shawry's up and about, he's in our back six," Armstrong said.
"Him being out does open the door a bit, but at the same time you don't get given games at this footy club.
"I feel I've been playing my role, which is all I can do. I have to not worry about selection and just do what I can to help the team win."
Despite his recent run, the 23-year-old knows he still has plenty to work on to establish himself in the team.
Blessed with natural kicking skills, defensive prowess is the key to Armstrong's longevity.
"The defensive aspects of my game are things I work on really hard," he said.
"I back my kicking and disposal efficiency to be up and crisp all the time, but that's not what gets me a game.
"It's the defensive side."
Armstrong works closely with Shaw, his mentor from last season, and Swans co-captain Jarrad McVeigh.
There are few better role models as Armstrong strives to adopt the work ethic required to taste the same kind of success as that pair on the last Saturday in September.
He describes watching last year's Grand Final as "like having a shower with a raincoat on".
"You were there, but you weren't quite experiencing the whole thing," he added.
While he understands the importance of discipline, Armstrong did break one rule that day.
"As soon as the siren went, we were running down and they were actually trying to tell us that we couldn’t go out there," he said.
"But I was going out there. We just jumped the fence and I think the first bloke I saw was Shawry.
"I just gave him a massive hug and it was just hugs all round. It was great."
Next time, he just wants to be a little bit closer to the action.
James Dampney is a reporter for AFL.com.au. Follow him on Twitter: @AFL_JD
