“It’s been crazy — a complete 180, to be honest,” Krug said in a phone interview Wednesday. “The lifestyle is so much different in pro hockey than it is in college hockey. It’s something that I have to adjust to, but the people I’m surrounded by have really helped me make a comfortable transition.”

Following a practice on Monday, Bruins head coach Claude Julien gave the green light and penciled the team’s new addition in the lineup for Tuesday’s game.

Paired with Bruins defenseman Johnny Boychuk, the 5-foot-9 former Spartan saw his first career NHL action against fellow Eastern Conference powerhouse Pittsburgh Penguins.

With a host of family watching live and a group of his former Spartan teammates watching the game on NBC Sports Network, the Livonia, Mich., native Krug totaled more than 18 minutes of ice time and got off two shots in his professional debut. However, the Penguins — led by the hot hand and a pair of goals by forward Sidney Crosby — beat the Bruins 5-3.

But for Krug, the experience of getting his first NHL action will be invaluable as he continues to learn the Bruins style of play and grow as a hockey player.

“Ever since I was a little kid and I could stand up on skates, my dream was to play in the NHL,” Krug said. “It was actually a surreal experience being on the ice with Crosby and (Evgeni) Malkin and James Neal and Kris Letang. … It was a great experience.”

One of the more supportive followers of his journey to the Bruins has been his former head coach Tom Anastos. Anastos flew to Boston last week to catch Krug warming up before the Bruins took on Alexander Ovechkin and the Washington Capitals.

After watching his former captain’s announcement to go professional in March, Anastos said it’s no surprise Krug has assimilated so well into the Bruins locker room.

“He’s doing all of the right things, which isn’t a surprise to me,” Anastos said during the game Tuesday. “I know he’s talked to a lot of the players to try to learn how to be a pro, and he’s a very observant guy. He’s watching, he’s studying how the Bruins play, and he’s trying to learn as much as he can in a short time to get himself into a position to have a chance to play.”

With his performance in his debut, Krug impressed many including Julien.

Of Krug’s performance, Julien complimented his new defenseman’s mobility and puck movement, among other things. Small criticisms of Krug came in adjusting to the Bruins style of play, but that’s to be expected of a new player in Krug’s position, Julien said.

“There was no hesitation once his mind was made up, and that will only get better as it becomes second nature, and that’s totally, as I said, totally natural for a guy playing in his first game,” Julien said in a statement released by the Bruins. “But the rest of it — as I said — when he had the puck, didn’t hesitate, thought he moved it well and made great passes.”

Krug likely will have many more opportunities at the professional level to make an impression with the fans of Boston. But for Krug, there will be few moments ever to compare to his first one.

“It was a surreal moment for sure,” Krug said. “You know, I still don’t know if I can fathom it or put a finger on it. I tried my best to remember as much as I can about the game. It’s something that I’ll remember forever.”