Penguins Coach Mike Sullivan has helped Pittsburgh turn its season around and become a Stanley Cup contender. (Gene J. Puskar/Associated Press)

The changes occurred over more than a month’s time, and eventually they altered the entire trajectory of the Pittsburgh Penguins’ season. But when forward Tom Kuhnhackl walked into the team’s practice facility Jan. 7 for the first time, “you didn’t know how to act around these guys,” he said. “It was really nerve-wracking.”

Here he was, a German-born player about to make his NHL debut on a roster full of household hockey names. Except the Penguins were scuffling. They were less than a month removed from firing former coach Mike Johnston on Dec. 12 and elevating former Boston Bruins Coach Mike Sullivan, who was just 23 games into his tenure as the head coach for the Wilkes-Barre/Scranton Penguins of the American Hockey League.

But Sullivan’s arrival also ushered in a wave of new faces from the minor leagues, and the infusion of this AHL flavor played an important role as the Penguins transformed into the NHL’s hottest team. They remain a tough out as their Eastern Conference semifinal series against the Washington Capitals returns to Pittsburgh for Game 3 on Monday night tied at one game apiece.

“They came into our team at a really critical point,” veteran Matt Cullen said. “They all helped us get to the level we’re at now, and some of them are still here playing at this level. They just made us a better team, and it’s good to see these guys taking advantage of their opportunities and establishing themselves as NHLers.”

The Penguins were 15-10-3 at the time of Sullivan’s promotion, and they would go on to lose his first four games as head coach. But a shift, both in personnel and philosophy, was going on behind the scenes.

Two days after Sullivan took over in Pittsburgh, the team acquired veteran defenseman Trevor Daley in a trade with the Chicago Blackhawks. A day later, the Penguins called up forward Conor Sheary and goalie Matt Murray to the NHL for the first time. Three weeks after that, Kuhnhackl and forward Bryan Rust arrived from the AHL. Pittsburgh then traded for forward Carl Hagelin on Jan. 16 and brought up former first-round draft pick Derek Pouliot from Wilkes-Barre/Scranton two days later.

So while stars such as Sidney Crosby and Evgeni Malkin remained, their supporting cast had been reloaded on the fly.

Sheary, for instance, is playing on the top line alongside Crosby. Murray emerged as the franchise’s goaltender of the future filling in for injured starter Marc-Andre Fleury down the stretch, and he won for the 11th time in 12 games in Pittsburgh’s 2-1 victory over Washington in Game 2 Saturday night. Kuhnhackl and Rust are key figures on the Penguins’ fourth line and penalty kill.

“When we first came up here, we were just trying to make our mark,” Rust said. “We knew what Coach Sullivan wanted us to do. He knew what type of players we were, and we knew that and we tried to take advantage of that and kind of run with it. We were just trying to have as much fun as we could, bring as much energy as we could, play as hard as we could, and it became a little infectious. That workmanlike attitude maybe rubbed off on some guys, and it helped us moving forward.”

Sullivan’s role can not be diminished, and the tenets of defensive accountability and puck possession that he preached in the minors did not change when he returned to the NHL. He wanted a “harder” brand of hockey and, like the players that ultimately joined Sullivan from the AHL, it added a new wrinkle to a locker room that “knew we had a lot of skill and we were hoping that would be enough,” forward Eric Fehr said.

“I had a real short-term focus,” Sullivan said this week. “Where that was going to take us, we really didn’t concern ourselves with that.”

The results are undeniable at this point.

Pittsburgh won 18 of its final 23 games this regular season to finish 48-26-8, emerging as one of the NHL’s fastest teams with a mix of established stars and new faces that came together just in time for a Stanley Cup playoff run.

“The longer we’ve been here,” Kuhnhackl said, “the more comfortable we feel.”