Since the moment the Czech was drafted by the Bruins 25th overall in the first round in June, he started his climb in Black & Gold.
Putting on the Spoked-B for the first time was just the beginning for the flashy, skilled winger.
The club's development camp in July was another step. He made enough of an impression that the Bruins wanted to bring him back to Boston for rookie camp in September.
On Tuesday, Pastrnak completed that step, scoring his first goal en route to Boston's 4-2 win over Nashville in the final game of the rookie tournament in Antioch, Tenn. His dynamic play was on display, earning him all-tournament honors from team officials and scouts.
Now comes main training camp and then preseason games, but Pastrnak is not getting ahead of himself.
"I don't know, I feel like I'm on second floor, and NHL is on fifth," Pastrnak said following the game, when asked where he feels he's at after rookie camp.
"So I have three more floors," he added with a smile.
That analogy didn't come from anyone except Pastrnak, but General Manager Peter Chiarelli and the Bruins' brass have the same approach for the forward: to manage expectations.
"Step by step," Chiarelli reiterated during the rookie tournament in Tenn. "Like I said before, he'll get through this [rookie] camp and then he'll be in main camp and we'll put him with some folks, so to speak, go up the ladder and we'll see how he performs."
"When you start playing against big, strong guys, the game changes, so we don't have any other set plan than to progress slowly."
At training camp, Pastrnak could find himself playing on David Krejci's right wing.
Entry-level free agent Reilly Smith remains un-signed, with the Bruins in a salary cap crunch, and if he does not report for the start of camp, it could open up more opportunity for young players like Pastrnak.
"Listen, if he's going to make our team, he's going to have to play higher up in the lineup. He's a skilled guy with speed and he needs to play with skilled players," said Chiarelli. "So maybe when I say he's not going to start down the lineup in preseason and camp, it's just - he'll have to be with skilled players because then you're not going to get what you want out of him."
"Who knows, he might start with Krejci, I don't know."
If Pastrnak and Krejci do get paired up, it would be special for Pastrnak, who grew up idolizing the fellow Czech.
"Honestly, I haven't even thought about it," Pastrnak smiled. "It will be great if it came true, and I just will try to do my best, and we'll just see what is going to happen."
"It's going to be nice, it's going to be really weird in the beginning, but I'm sure David's going to help me and whether we're going to play together or not, he's just going to try to help me feel and make me comfortable."
Even if Pastrnak does not start out camp playing with Krejci, he'll still be placed with top players.
"It's about getting him with skilled players, and measuring the expectation level," said Chiarelli. "I mean, everything's about him right now. You know, there's other good players out there too. Like [Brian] Ferlin's a player, I thought he was outstanding [in rookie camp], he's just not as flashy as David, so there's other players."
"But with David, let's just take one step at a time - and a smaller body can wear down over time, so let's see how he can handle that stuff."
At 170 and change, the 18-year-old Pastrnak doesn't have the advantage of size, and that will be a test for him moving forward into main camp, battling the likes of Zdeno Chara, Johnny Boychuk and Adam McQuaid.
He'll need to show he can win puck battles on the walls, and get the rush started up ice.
"It's not all just the speed part of the game and the individual puck skills; it's the grunt work," said Providence Head Coach Bruce Cassidy, who served as the bench boss during rookie camp. "So [Boston will] get a look at that obviously during exhibition season and go from there."
"But physically, he seems fine. He gets chipped and bumped here and there, but he's in and out of traffic and seems to have a way to keep his head up - he didn't get drilled, for a guy that is certainly willing to cut to the middle - so, so far, so good in that area."
If you ask Pastrnak, he doesn't seem too worried about his size, either.
"I haven't thought about it yet. I've been focusing for the games here [at rookie camp], so I wasn't thinking about it yet," said the forward, who often doesn't dwell too much on anything except "just playing hockey."
While speed and skill are Pastrnak's assets, he's learning it's much tougher to make plays in the North American game. The pace is faster. He had more time in Sweden with Sodertalje SK.
He's also experienced a learning curve in the defensive zone, away from the puck - a universal learning curve for all young players, especially in the Bruins' heavily two-way system.
Pastrnak takes a lot of pride in hunting down players on the backcheck, but he has work to do when it comes to the "risk/reward" part of his game (as Chiarelli and Assistant General Manager Don Sweeney have labeled it). He needs to understand the right moments to be aggressive in the defensive zone, and when to hold back.
"Every game, you're getting better with more experience, and I'm just trying to get as much experience as I can," said Pastrnak. "And now I know I have to be more simple in the defense zone, and I didn't know this two months ago."
"So I think every game, you have to take some experience of the game, and not always you have to take just the positive, you know, always you have to think what you did wrong and then next time make it better."
"It's not like we're going to protect anybody in these situations," said Sweeney. "We're going to have him experience as much as we can."
As Pastrnak approaches main camp on September 18, he's filed away the experiences of development camp and rookie camp. Chiarelli and the Bruins' brass have as well.
Now, the climb continues.
"Well there’s a progression, right?" Chiarelli said back during development camp, during step one. "There’s the camp: bigger, stronger, faster. There’s the preseason: bigger, stronger, faster. And you’ve got those nine games - again, bigger, faster, stronger regular season."
"It’s another test, another test, and then you make the decision."