Henderson has taken an unlikely path to lead Ulstermen
The skinny, shaggy-haired kid with the Belfast Royal Academy shorts and the knee injury sat on the wall as Ulster's senior squad ran lengths of the pitch, unable to train thanks to an ill-timed patellar tendon issue.
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Not sure what the story was with their latest Academy recruit, Rory Best certainly wouldn't have believed he was looking upon the man who just a little more than nine years later would succeed him as the Ulster captain.
"Some judge of talent," he would later joke.
"If you'd told me he'd go onto be the player he is, I'd have laughed at you," adds Darren Cave, Ulster's most capped player who also retired last summer. "He was gangly, basically an over-sized schoolboy with stupid hair. What he's developed into, he's turned into a fantastic player and a fantastic leader."
Henderson, who was told of the captaincy by head coach Dan McFarland one day before joining up with Ireland's World Cup camp, will lead out the side for a first time in his permanent role today when taking on Bath in the Champions Cup. He will do so having been the obvious and outstanding candidate to fill the considerable void left by Best after 15 years, a rare homegrown star possessing more than 50 Test caps and the experience of a Lions tour to New Zealand under his belt.
His route to this point hardly seemed pre-destined though.
Born into a family steeped in Academy Rugby Club - now on the walls of clubhouse in north Belfast, where his entire extended family gathered to watch the Japan World Cup clash, hang a pair of Henderson's Ireland and Lions jerseys - his father Gordon, a quantity surveyor, had captained the club for the 1987/88 season when they were in the senior ranks.
It was there that he played mini-rugby growing up, his first run-out at Kingspan Stadium coming in the colours of Ben Madigan Prep at a primary school festival. The squad picture that day, with Henderson at least a head taller than most of his classmates, shows that he always had the raw attributes to succeed and he would show promise in athletics too, most notably throwing the discuss.
Teachers at Ben Madigan recall a relaxed child with a head for numbers, the laidback nature that would one day see Donncha O'Callaghan christen him "the llama" evident even then. Those who coached his earliest rugby, however, noted that he was a different character on the field, finding an aggression that was hitherto latent even if his older brother's arm still bears the scar of trying to sneak a piece of steak past his plate at one family dinner.
Moving onto B.R.A, the alma mater of Grand Slam winning captain Jack Kyle, his talent was unquestioned, although to describe him as a star of the future would be a stretch.
"He never played any representative rugby or anything like that, for Ulster or Ireland," says Chris McCarey, coach of the B.R.A team that with Henderson and Stuart Olding went all the way to the Schools' Cup final of 2010.
"He had an injury that meant he didn't play at all in year 13 so he was probably off the radar a bit.
"He came back and we played our first game of the season against Ballyclare High. He picked the ball up in our '22' and didn't get tackled until he was on their five-metre line, it took that many of them to drag him down.
"I remember just looking at David Creighton who was coaching with me and it was sort of like, 'oh my goodness, we might have something special here.'"
The opposing coach that day was Dan Soper, now Henderson's skills coach at Ulster.
"He certainly stood out," remembers Soper. "Well, he had that long hair at the time too. No, he was definitely somebody that you'd remember and then follow how they were tracking."
Others remember a ferocious counter-ruck against RBAI late on in their upset win at Osbourne Park to set-up the winning try, a quintessential man against boys moment.
And yet, having lost that Schools' Cup final to Ballymena Academy, that was almost that. An offer to study actuarial studies at Heriot Watt had been accepted. He wasn't quite at the docks by the time Jonny Bell intervened with an offer of an Ulster Academy place but not far off. An about turn underwent, instead it would be Queen's and Ravenhill. Despite the initial scepticism, he wasn't long making an impact.
"Well, once we saw him tackle Stephen Ferris in training we knew he was different," says former Springbok Stefan Terblanche, Ulster's starting full-back when Henderson made his debut from the bench against Connacht in April of 2012.
Just seven months later, he was making his Ireland debut against South Africa in Dublin, reflecting afterwards that it had not been long since he'd been living in a student house melting whatever was to hand in the microwave.
In those early days, time management and homework weren't particular strengths. Team-mates would laugh at the level of fame enjoyed by opposition players of whom Henderson would be blissfully unaware, but soon noticed a transformation. Clever enough to recognise what it would take to reach his potential, he began to mimic the more dedicated senior pros while many noted a maturity beyond his years aided by a settled life at home after he married childhood sweetheart Suzanne Flanagan in 2017 with the couple now expecting their second child.
Following the example set by Chris Henry - "the two sleepers, he taught him to nap," jokes Cave - and then Rory Best, Henderson quickly became more rounded, relying not just on the freakish strength that once saw him stop McCarey, himself a number eight once on Ulster's books, dead in his tracks with one arm at a school training session but rugby smarts too.
His influence on the squad grew too, the type of quiet leader who noticed a young player without the right type of boots and turned up to training with a box-fresh pair for him the next day.
And while he may not be the speaker that Best was before him, his own style has a way of lifting those around him too, whether it be in the big moments he is able to make over the course of 80 minutes or by setting the right example for others to follow. On his first day back following the World Cup last week, the squad's young players and Academy men walked past the gym to see their captain putting in an extra session of catch and pass drills in the gym.
"He's a man of not too many words but on the pitch he has that aura of confidence about him," says Kieran Treadwell, who has shared the engine room with Henderson over the past three seasons.
"He's a big presence and when the going gets tough, he puffs his chest out and you want to do the same."
For all he's already achieved, this year feels a big season. Having shouldered an increasing load for both Ulster and Ireland, he comes back from a World Cup where he was brilliant against Scotland before, like Joe Schmidt's side on the whole, his tournament slipped from that high watermark.
At just 27, this can be seen as the beginning of the second act of his career, what happens from here will decide whether he goes down as a very, very good Ulster player or a true great of the province.
Those that have played with him remain in little doubt over how he will be viewed when all is said and done.
"When I first came to the northern hemisphere, it was at Ospreys," remembers Terblanche.
"At that time there was a young guy coming through who reminded me of what I'd later see with Henderson - Alun Wyn Jones.
"That's the type of career I think he could have in the end."