A double compound fracture of the leg while playing in a reserve game for Crystal Palace brought a nightmare end after he had achieved his "boyhood dream" of becoming a professional footballer.
Harrison was plunged him into the depths of despair.
Such was his depression he spent two Christmases by himself, eating baked beans on toast, but thanks to his family he came out of it and turned his life around.
Now 46, the humble Harrison has become the Welsh league champions' most decorated manager over two spells -- 17 trophies in all including winning the league title last season without losing a match.
"What does not kill you makes you stronger," Harrison tells AFP, looking back to the dark days.
On Tuesday, instead of watching the Euro 2024 semi-final between France and Spain he will be pitchside at New Saints' home ground in Oswestry, Shropshire, as they host Montenegrin side Decic in the Champions League qualifying round first leg.
It will be his 20th Champions League game, edging him closer to breaking the record for the number of matches an English manager has taken charge of in European club football's premier competition.
That belongs to the late Bobby Robson, who was in the dugout for 26.
Harrison though is no stranger to breaking records held by icons of the sport.
In 2017, when the New Saints won 27 successive matches, they broke the mark held by the great Ajax side, including Johan Cruyff, for victories by a top-flight side.
"Yeah it is fun, bit tongue in cheek," chuckles Harrison, a former Middlesbrough teammate of Paul Gascoigne.
"Myself and the football club are not in the same league as that fantastic Ajax team or Cruyff and it is nice for me and the club to be spoken about in the same sentence.
"We are obviously doing something right.
"It would make a good question in Trivial Pursuit in the future!"'Bit of adversity'
As Harrison eyes round five, the New Saints' ebullient owner Mike Harris is aiming higher, though all in due time.
"To win it," Harris tells AFP.
"Getting to the group stage, I never dared say that a few decades ago. That is a natural ambition.
"Get out of that and then into the final 16 and hopefully over time keep knocking at the door and eventually it will break.
"It is the dream that makes football exciting."
There is nothing more Harrison would like to achieve for a club "I love and an owner I love working for."
Harrison, who learned about managing from playing under Bryan Robson and former Manchester United and West Ham manager David Moyes, remains grounded.
"Obviously Mike is really passionate about the New Saints being the first Welsh club to get into the group stage of the Champions League," said Harrison.
"But that is never going to happen. The Europa League is a huge ask, though the Europa Conference League is not unrealistic."
Harrison and Harris make the perfect double act, the former's more modest ambitions a balance to the latter's romanticism.
Harrison is no killjoy, just moulded by his bitter experience along the way, including a dreadful spell managing Hartlepool.
"Make no bones about it I learned how to be a better manager and a better, more rounded person with a bit more empathy," he said.
"I became a better partner to my wife Danielle and a better dad to my daughter Ruby."
He says the most upsetting thing about the Hartlepool nightmare was how sad Ruby, then four, was about the upheaval of moving to a new home and school.
He adds, though, just as adversity forced him and Danielle, a successful ballet and West End show dancer, to grow up, so the Hartlepool experience will benefit Ruby.
"If you get to adulthood without adversity, what is going to happen when you first meet it?" he said.
"Adversity makes you a more whole and rounded person."
