The transaction came as the Bruins made a number of roster moves before submitting their 23-player opening day roster.
"We’ll have a little more time to watch him and assess him," said Chiarelli.
Pastrnak didn't have luck on his side at the start of training camp, tweaking his shoulder on just the second day of on-ice sessions by going awkwardly into the boards.
He only had the chance to suit up in the Bruins' two final preseason games.
The 18-year-old is also adjusting to the North American style of play, with much less timing to make plays. It's an adjustment that he can handle, but one that is always a transition for European players.
"Those were really the two reasons," Chiarelli told reporters during a press conference at TD Garden on Tuesday. "And he’s 170 [pounds] soaking wet and he’s 18. I thought he played well as the camp went, and when he recovered from his injury and once he got going."
"He’s got to get better, but it’s more about giving him time that he didn’t have for evaluation in camp based on his injury and acclimatized to that North American pro style. It’s an ability we have and an opportunity we have to do it. We thought that we’d put him down there [in Providence] for a bit and see how he does, and assess later on."
With Pastrnak a short drive away in Providence, Chiarelli and the Bruins' brass will be able to watch him progress during the next few weeks.
Once they feel they've gotten a strong enough evaluation, they can further discuss the options for the first rounder.
Ultimately, Pastrnak could come back up to Boston, stick in Providence for the season, or be loaned back his team in Sweden, Södertälje. If he's not with Boston, Pastrnak has the right to go back to his Swedish team.
Since he is a first round pick under the Swedish Transfer Agreement, Chiarelli said the forward could possibly play the entire season in Providence and still have the NHL entry-level contract he signed this summer slide to next season. If he plays more than nine games at the NHL level in Boston, the contract would kick in.
The best option for Pastrnak will eventually be determined, after discussions among the forward, his agents and the Bruins.
"Us not having that full assessment period - that was really that’s the driving force behind it," Chiarelli said. "I don’t want you guys to read too much into it, but we'd like to see him down there and play some games."
"It’s a good opportunity to have him on our soil and closeby to watch him. In two or three weeks, we will look at him and make a decision as to what the next step is."
"The 18-year-old European player in the American League is not historically—more for acclimatization reasons, it's hard. Long story short, we’ll have a look at him in a short period of time then we’ll have a decision."
On Tuesday evening, the Bruins also announced other transactions in accordance with the opening day roster.
Jordan Caron was assigned to the Providence Bruins, while Brian Ferlin and Malcolm Subban were recalled from Providence.
David Krejci was placed on Injured Reserve, Marc Savard was placed on Long-Term Injured Reserve, and Gregory Campbell and Anthony Camara were placed on Non-Roster Injured Reserve.
Chiarelli had alluded to these transactions earlier on Tuesday.
"I don’t know what roster comes out through CR [NHL's Central Registry] or they may control our website roster, so the roster you see at 5:00 p.m. [on Tuesday] won’t necessarily be the roster you see [Wednesday]," said the GM. "When you’re into the long-term injury exception, you have to do some roster manipulation to properly invoke it and maximize it."
Krejci's undisclosed injury was originally deemed "very minor" following Saturday night's preseason finale. He was not on the ice with the team for practice on Tuesday, and it was later announced that he was placed on IR.
"It hasn’t healed as well as we thought it would," Head Coach Claude Julien said Tuesday.
Campbell was sidelined for the majority of camp with a mid-core injure and just recently started skating. Both Campbell and Krejci will now miss at least the first few games to start the season.
Simon Gagne remains with the team for the near future on a professional tryout agreement. Without a contract, Gagne is not listed on the roster or eligible to play in games.
"He's going to stick around and he’s going to continue to skate with us and travel with us," said Chiarelli. "He’s getting better and we’ll see where it goes in a short period of time."
"I don’t want to give you everything. He likes what he’s seeing and certainly a player that if he can get close to what he was before, he’ll be a helpful player."
Notable names on the Bruins' opening day roster include Craig Cunningham, Matt Fraser, Bobby Robins and Ryan Spooner. While Spooner, Fraser and Cunningham were all called up to the big club last season, Wednesday night would mark Robins' NHL debut if he suits up.
Cunningham could slot in for the injured Campbell in the short term. After a late start to camp due to recovery from mid-August mono (in which he could do nothing for three weeks), Cunningham got himself up to speed quickly enough to earn his spot.
"We feel with Campbell’s injury right now and different things, [Craig's] a guy that we have to have a serious look at," Julien said prior to the roster being submitted. "He’s certainly a guy that’s paid his dues down there, he’s worked hard and he’s earned that opportunity here."
Robins has paid his dues, and then some. Prior to spending the past three seasons in Providence, the 32-year-old bounced around teams from juniors, to college, the ECHL, AHL and overseas for more than a decade before getting this opportunity.
Fraser has utilized his quick release to put up offensive numbers at the AHL level for the past three years. A standout during camp with not only his shot, but his grit, Fraser will look to establish himself in an NHL role.
Spooner will also look to solidify himself with Boston, after a season of ample callups that resulted in 23 games. The speedy forward has well-known offensive touch, but will work to finally find the back of an NHL net, while proving to his coaches that he's reliable in defensive situations. Learning to play on the wing will help his versatility, but with Krejci on IR to start to the season, he'll get the look at center.
"He’s an exciting player, he’s got good speed and he’s got a lot of things but we all know that in this league if you’re a liability it’s hard for any coach in this league to be able to have confidence in putting him on the ice," said Julien. "Sometimes you have to learn your trade in the minors when you don’t get it. Right now, I think he’s got it."
"And if he can hold onto it, we're going to have a great player. He has great potential but, again, there comes a time where it’s in his court and he’s got to take advantage of it, and we’re giving him that opportunity right now."
The Bruins' 23-man roster as of the morning of October 8 is as follows:
FORWARDS (13): Patrice Bergeron, Craig Cunningham, Loui Eriksson, Brian Ferlin, Matt Fraser, Chris Kelly, Milan Lucic, Brad Marchand, Daniel Paille, Bobby Robins, Reilly Smith, Carl Soderberg, Ryan Spooner
DEFNESEMEN (7): Matt Bartkowski, Zdeno Chara, Dougie Hamilton, Torey Krug, Adam McQuaid, Kevan Miller, Dennis Seidenberg
GOALTENDERS (3): Tuukka Rask, Malcolm Subban, Niklas Svedberg