Thought it was clear that the talent was there in 2015-16 -- or earlier, when Pastrnak was selected by the Bruins with the No. 25 pick in the 2014 NHL Draft -- it's never quite as clear what a player is going to do with those raw natural gifts.
"You do never know how a talent like that is going to play out," Marchand said. "He was 18 when he first started, and when you have young success and you're as talented as he is, you don't always need to put the work in. I think that's where a lot of kids, they come up on the talent and they don't necessarily need to work as hard to get in the League -- but once you get here it's a completely different animal."
That was why those two months back in 2015 were so crucial.
"I think a big thing -- and if you ask him, he'd probably say the same thing -- he spent a lot of time with Chris Kelly when he was hurt, the year Chris was hurt," Marchand continued. "I think that he got to see what a true professional is like when they're out of the lineup and how hard they work."
For much of the time, it was just Pastrnak and Kelly, working with then-strength coach John Whitesides. They were doing what they could while their bodies healed, working on building fitness and strength, a daunting task when the days out of the lineup stretch from weeks to months. It was especially daunting for Pastrnak, given that Whitesides was particularly tough on younger players.
But Kelly recalled that Pastrnak was so dedicated and worked so hard that Whitesides took it easier on him than many of his compatriots.
"Speaking with John when [Pastrnak] maybe wasn't around, I think at training camp he maxed out on his bench press at like 150, and then by the time he went back to playing, being with us that short period of time, I think he was up to like 185, 190 on the bench," said Kelly, now the player development coordinator for the Bruins.
"Just the work ethic he put in in that short period of time was just, it was so refreshing."
Part of that was learning from Kelly, a hard-working third-line center who served as an alternate captain during his time with the Bruins. His message was simple: Don't waste this time. Work.
Pastrnak was receptive.
"If you're going to be honest, you never truly know how a young player is going to develop and what type of person and player is he going to become in five or six years," Kelly said. "When he first came in, there was a lot of junior habits in his game, where he was trying to beat players one-on-one. We had an older team at the time, and I think the guys were obviously wanting to help him and would suggest things.
"And then the next year when he came in, it was like he had learned a lot. He can't play the game one-on-one; he has to use his linemates. And he really learned the pro game that much more."
He had listened. He had taken what they said to heart. He had worked on his game.
"I'm just so young and there is every single day, whatever you do, even if you do a little bit of extra, you can always get better," Pastrnak said. "There's so many details in hockey. Hockey is such a crazy game that you never get to that point that, OK, there is nothing I can do badly. I don't think you can get to the point that there's nothing you can get better at."
This season has continued to prove that for Pastrnak, who scored an NHL career-high 38 goals in 66 games last season. This season, he is better.