The following week, a small, non-displaced fracture turned up in the images of Pastrnak's foot. He wound up missing the next two months, a lifetime for a 19-year-old with 56 NHL games played to that point. Kelly would miss the remainder of the season.
On the surface, the two injuries would seem to have nothing to do with each other, just quirks of fate for two forwards playing on the same team. It left the Bruins shorthanded, sure, but it would have been nearly impossible to predict that the effects are being felt four seasons later, now that Pastrnak has become one of the best -- and certainly the hottest -- players in the NHL.
Pastrnak, who is set to face the New York Rangers in the 2019 NHL Thanksgiving Showdown at TD Garden on Friday (1 p.m. ET; NBC), leads the NHL this season with 23 goals in 25 games, five more than linemate Brad Marchand and center Connor McDavid of the Edmonton Oilers.
Video: BOS@MTL: Pastrnak scores in 2nd period
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.
Video: Enterprise Hat Trick: Pastrnak nets 3 vs. Canadiens
"It's his compete on the puck, everywhere," linemate Patrice Bergeron said.
Said coach Bruce Cassidy, "I think he's hitting his one-timer cleaner this year, so obviously his execution of his shot is better."
It's the little things, and the big things.
"Just watching his confidence and his abilities and the way he controls games now, he's on a different level than he was last year, and he was good last year," Marchand said. "But he seems even more confident this year and I don't know if it's a year old, a year wiser, stronger, what it is. I'm not going to be able to put a finger on it.
"But he controls games and he controls plays. You can put him one-on-five out there and he'd still make a play happen. It's great for us and it's fun to play with. … There's not many young kids in the League that are as talented and as strong and dedicated as he is - and a great person."
Asked if he could get to 50 goals -- something that has happened in the NHL 28 times in the past 20 seasons, and four in the past five seasons - forward David Krejci said, "I hope so.
"He's on pace for what, 70?"
Actually, Pastrnak is on pace for 75 goals. No player has reached 70 since Teemu Selanne and Alexander Mogilny each scored 76 in 1992-93.
"It's a big motivation for me," Pastrnak said of scoring 50 goals. "I want to get there and I will get there one time when I just need to be healthy and work on my game. That's what I'm doing, so that's why I think I'm very capable of getting there. I just need to stay healthy and need the same work ethic every day I come to the rink.
"That's the only thing I'm focusing on, to get better."