Armstrong's day started when he left Melbourne for Geelong at 7.30am ahead of the Magpies' VFL side's game at Simonds Stadium.
He was one of the Pies' best in the 14-point win, which started at 11am and sewed up their spot in the VFL finals.
He stiffened up on the drive back, went home for a quick feed of leftover spaghetti bolognese, sat on the couch for 20 minutes and then headed to the MCG to fulfil his obligations as second emergency for the Pies' clash with the Brisbane Lions.
He did all this not knowing Jamie Elliott wasn't going to play after developing hamstring tightness and that Clinton Young was going to appear in his place.
This left Armstrong as the next cab off the rank when captain Scott Pendlebury went down with a quad injury in the warm-up.
"(I had) eight or nine minutes, maybe," Armstrong said, when asked how long he had before he knew he was playing.
"I was down here (in the rooms). (High Performance Manager) Bill Davoren said you can almost head home and get a bit of rest and the next person who came in the door was someone telling me to kit up.
"I was coming prepared to play but not expecting to...knowing the game-plan and all that stuff, that was running through my head but the last thing I expected was to play.
"It was just a bit of a shock."
Armstrong ran 14km in the VFL on Saturday before having to back up on the AFL stage as the starting substitute.
What wasn't predicted was the fact he would be required as early as he was after being given the green vest.
Ben Reid's hamstring injury four minutes into the game meant Armstrong had just three minutes to shake off any lingering stiffness and take to the field.
Coach Nathan Buckley said the situation was unusual but no team expected to lose two players to injuries before the game.
"Philosophically we discuss what we do with the carry-over player every week. I think most teams will have a philosophy on that," Buckley said.
"We feel like we need to get games into our players when we can so we didn't carry one over today.
"Worst case scenario, we manage the game time of a couple of our emergencies and they're there to come into a sub role if we have a situation like tonight.
"I don't know what has to happen or how often it happens where that player goes into a sub role and he's into the game five minutes in because someone else gets injured. It's not something we plan for."
Armstrong estimated he was about 13 years old the last time he was called upon to play eight quarters of football in one day.
He wasn't the Pies' worst player – he tired towards the end as most players did when the injury crisis restricted the bench to one fit man – and he ended with 17 touches.
Now, he'll focus on recovery in the next few days as he tries to cope with the physical – and emotional – fall out of playing in one inspiring and important win that sealed a finals berth, and a loss that has almost ruled one out.
"I've pretty much see both ends of footy today. It would have been nice to get the eight points today but unfortunately only four," he said.
He also echoed Buckley's thoughts that the Pies won't write off the rest of the season despite the precarious position they are now placed in outside the top eight on percentage.
"All we can do is keep playing the best footy we can - we just need to try and finish the year off strongly," he said.
"If that's two weeks, if it's more, whatever it is – we've just got to do our best and do the Collingwood faithful proud."
