The NHL gathered this past week in Las Vegas to stock its newest expansion team, but the real gambling will take place Friday at the annual draft in Chicago.

As fans in Edmonton and Toronto can attest, there is no other day on the hockey calendar that can so profoundly shift a team's fortunes. Over the past two years, teams with high-end picks were spoiled by the immediate impacts made by Connor McDavid and Jack Eichel, Auston Matthews and Patrik Laine. All four exceeded the lofty expectations placed on top prospects.

But this is a largely unprecedented phenomenon – that consecutive entry drafts produce a quartet of potential generational players. And it means there could be a McDavid-Matthews hangover at this year's draft, which doesn't offer the same star power as the previous two.

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It is unlikely that Nico Hischier (Halifax, Quebec Major Junior Hockey League) and Nolan Patrick (Brandon, Western Hockey League), the top two projected picks, will be able to realize their potential as quickly as McDavid or Matthews.

According to former National Hockey League general manager and long-time scout Craig Button, this exposes the biggest challenge scouts face: imaging how a teenage prospect will evolve into a full-fledged adult pro.

"Here's the example I would use," Button says. "If you wanted to become a lawyer, you first go to high school, then to university and then to law school. At that point, you article with a firm, where they get to test you and you get to see what it's all about – and then they either offer you a position or they don't.

No self-respecting law firm would ever promise a future partnership to a talented high-school student, but that is effectively what the NHL does, Button says.

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"At 18, we hand you a massive paycheque and then at 19, we're asking you to argue a case in the Supreme Court – because that's what the NHL is, the Supreme Court. And you're not just playing against the best players in the world, you're also competing against players that are three and five and eight and 10 years older than you, that have crafted their games. So you're not only arguing a case in the Supreme Court, but across the aisle from you, you're facing Johnnie Cochran."

Button, who was scouting director for the Dallas Stars when they won the 1999 Stanley Cup, does not believe a player's failure to meet expectations right away is that player's fault. He says there is no single reason why highly rated prospects don't always turn into franchise players.

Drafting is inexact, more art than science. Moreover, the pressure to perform is greater on highly anticipated players; the complex human condition is inherently difficult to predict; and the ability of people to channel fear, hope, expectation and their competitive urges varies greatly from individual to individual.

In the same way it is difficult to predict why someone such as Nail Yakupov, chosen first over all in 2012, is struggling to find his way in the NHL, it is equally hard to forecast why a Johnny Gaudreau, 104th over all in 2011, is already an established star.

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"There is so much learning that needs to go on, but now with the salary cap you need younger players and so teams want to rush players to the league," Button says. "But it doesn't necessarily mean players are ready.

"I just did a top-50 affiliated players list and I had Jesse Puljujarvi (third over all in 2016, behind Matthews and Laine) on top. Just because he's not in the NHL, are you telling me Puljujarvi isn't a really good prospect? He is. He's just not ready right now. But those are two different things.

Can't-miss prospects who miss

In 1992, Todd Warriner was the fourth-overall choice in the draft, a year before Alex Daigle went to the Ottawa Senators as the much-heralded first pick of 1993. Whenever the notion of a draft bust is discussed, Daigle's name frequently tops the list. Warriner, a contemporary of Daigle and now a hockey commentator, says if Daigle came around today, he would probably succeed – because the traits he had, primarily his speed, are valued far more in today's NHL than they were two decades ago.

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"Daigle was a gifted player, who came from a junior league that played wide-open, 1980s style hockey to an NHL that all of a sudden was about the big strong guy and clamping everything down," Warriner says.

Daigle did himself no favours when he memorably stated on draft day: Nobody remembers who went No. 2. As it turns out, everyone does. It was Chris Pronger, who went on to have a Hall Of Fame career. At No. 4, Paul Kariya was a three-time 40-goal scorer who averaged more than a point a game in the heart of the NHL's dead-puck era. But after those two, the next best player was probably Saku Koivu, chosen 21st over all by Montreal. It was not a deep or particularly distinguished draft.

Button has spoken to Daigle a number of times over the years and believes the biggest single biggest miscalculation Ottawa made was not probing deeply enough into Daigle's personality, or his devotion to the game.

"If you talk to Alex, the first factor was, I don't think he had a driving, burning desire to play," Button says. "He was really good, but he didn't have that Sidney Crosby, every-day-is-a-new-challenge-for-me desire.

"Alex Daigle was also a perfect poster boy for why the NHL needed to fix the CBA. He got that huge signing bonus and he was resented by the players. They wanted to bury him. And he was on a terrible team. He scored 20 goals in his rookie year on a terrible team. I think the combination of everything led to where Alex was."

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The 20/20 hindsight on 2012

The best recent example of a scouting miscalculation may well be Yakupov in 2012, a draft that produced real mixed bag of early successes and failures. None of Yakupov, Griffin Reinhart at No. 4, Derrick Pouliot at 8 or Slater Koekkoek at 10 have established themselves as NHL regulars yet.

But Alex Galchenyuk at No. 3, Morgan Rielly at 5, Hampus Lindholm at 6, Jacob Trouba at 9 and Filip Forsberg at 11 are all exciting young players, who potentially still have more growth in their games.

If you redrafted 2012 five years later, you'd get a far different outcome. Hindsight is like that. Roughly half of the first top 12 are on their way up; and the other half are struggling to gain a foothold in the NHL; while a handful of others look like absolute steals – Shayne Gostisbehere at 78, Matt Murray at 83, Colton Parayko at 86, Frederik Andersen at 87, Jaccob Slavin at 120 and Ben Hutton at 147.

Button can muster some sympathy for Yakupov, who is playing for his sixth NHL coach.

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"The first overall pick in one year is not the first overall pick in another year. There isn't always a Connor McDavid or Auston Matthews there."

Mining for diamonds

Most scouts say it is harder to project success in the NHL and in Major League Baseball because players generally get drafted in their teens, whereas the National Basketball Association and National Football League tend to draft older players.

But to Vaughn Karpan, the director of player personnel for the expansion Vegas Golden Knights, one common thread exists. Just about anyone can identify players at the top end of the draft, even if some eventually don't pan out. But the key to successfully building an organization is wading through the second tier of prospects – to find potential diamonds in the rough.

"You look for those players that don't fit into a box, but somehow find a way to always be involved," Karpan says. "They're small. They're slow. They're whatever – but gosh, they keep playing. There are a number of them in hockey, but you find them in all sports.

"Jerry Rice, in football, had a legendary work ethic. But he wasn't just a fast guy running around a football field. He had to be smart. And he had to be determined. People who are willing to do those things are successful. Typically, in our game, it's been the smaller guy because he's had to adapt."

Karpan was the Western Canada scout for the Montreal Canadiens when they drafted Brendan Gallagher 147th over all in 2010. That was one they got right. But Karpan also remembers passing up the chance to draft Brian Gionta (82nd over all to the New Jersey Devils in 1998) because he was considered too small to play in the NHL. Gionta has played 1,006 NHL games thus far in his career.

"It's not enough to be just a small hard-working guy," Karpan says. "The minors are full of players like that. You've got to be special. You have to have determination."

Sizing up teens' prospects

Even in the modern-day NHL, with its emphasis on skill and skating, Button detects a lingering bias against smaller players.

Gaudreau, for example, he describes as a victim of "tape-measure scouting. It was the same with Theo Fleury. He was an unbelievable junior player, but he was 5-foot-6. I could show you my notes on Henrik Zetterberg. We had no problem identifying Henrik Zetterberg as a skilled, smart talented player. But when we went around the room, the reaction was the same from everyone: Too small."

Because the NHL drafts players who are usually still physically immature, the most challenging part of predicting the future is trying to project how a player might look when he is fully grown.

"What you learn is, sometimes you have to allow for the possibility that someone will have a growth spurt," Button says. "You look at these kids at 17 and it's easy to conclude they're weak and small. The larger question is, where do you think they'll be when they're 23? That's an important factor – physical maturity – and it can go both ways."

Button cites the example of David Chyzowski, chosen second over all in 1989 behind Mats Sundin. Button describes Chyzowski as "a man at 18. He could overpower goaltenders; he could shoot the puck past them and through them.

"But now you're in the NHL, where you don't skate by people any more and you don't overpower them. You don't just brush aside defencemen. You've got to be able to skate and think and make plays – and [Chyzowski] couldn't do it at the NHL level. Just because you're bigger and stronger in junior doesn't mean it will necessarily translate to the NHL.

"Then sometimes, as a scout, you just make the wrong assessment on a player. When we drafted Jason Botterill, we liked everything about him except his skating, but we thought that would improve. You know what happened? It never did – and he worked his ass off at trying to become a better skater. Is that his fault? No, it's ours. We made a projection – and we were wrong."

For most prospects not named McDavid or Matthews, the draft is a beginning, and the smart ones understand that the real work of becoming a professional is only now starting. But not everyone gets that.

"I've been involved with players who played hard to get drafted and then that was it," Karpan says. "They didn't really enjoy it and if you talk to them, 15 years later, they'd probably tell you that. But the biggest problem is [teams] want [prospects] to be something they're not. You have to look at them and see what they are and say, 'Can I use that?' – and then help them be as good as they can be."