Nick Montgomery has seen plenty of examples over the years: The guys who are told they'll never make it as a professional, only to defy the odds and make the coach who wrote them off look foolish.
The former Central Coast Mariners skipper reckons Usain Bolt might well prove to be the next one.
Bolt starts his "indefinite training period" with the A-League club on Tuesday -- coincidentally, his 32nd birthday.
It'll start brightly, with a huge media contingent expected at the Central Coast Stadium in Gosford for his first session.
But most football insiders predict the experiment will end without a contract for Bolt.
The video evidence from his previous stints in Germany, South Africa and Norway suggests the jovial Jamaican has a long, long way to go to realise his lifelong dream of playing professional football.
Montgomery, who carved out a decade-long career in England's top divisions, knows exactly what it takes.
He also knows that sometimes, expert predictions don't always prove correct.
"Players who people maybe don't think are going to make it professionally, a lot of times they prove people wrong, because they're the ones who've got the desire and the determination," Montgomery told AAP.
"For me, that always outweighs talent in naturally gifted players. I've seen it numerous times."
Now an assistant to Mariners head coach Mike Mulvey, Montgomery reckons Bolt has the best of both worlds -- a world-class athlete with the hunger and determination of an aspiring teenage footballer.
Montgomery and the rest of the Central Coast coaching staff sat down for dinner with Bolt the night he arrived in Australia, and he said he was struck by how genuine his desire was.
"It's definitely no joke for him. He's got a lot to lose," he said. "He's just a top guy, a really humble guy. He's just so determined to prove everybody wrong.
"To get to number one sprinter in the world, he probably had many doubters at the time that thought he couldn't do that.
"For the modern day footballer, number one, you have to be an athlete.
"So he ticks that box; he's a mountain of a man. But it's a different type of athlete to be a footballer.
"Only time will tell."