Suddenly, Torey Krug found himself under the spotlight.

Flashes of color began flying across TD Garden, the music started playing, and the sellout crowd of 17,565 was in full force.

And then the 5-foot-9-inch defenseman, who was not drafted by teams wary of his size but was signed by the Bruins as a free agent, stepped out for his first NHL shift, losing himself in the pace of the game for a few brief moments while he watched his dream come true.

The Bruins lost, 5-3, to the Penguins Tuesday night, but Krug, who skipped his senior season at Michigan State and signed with Boston March 25, didn’t disappoint in his debut.

“I thought he did a great job,’’ said Tyler Seguin, who is nine months younger than Krug and remains the baby on the Bruins roster, though he said he’s taken Krug under his wing.

“He’s not walking in against any average of the pack team, he’s going against one of the top teams in the league. And he seemed pretty cool about it.’’

Krug’s first shift was a bit clumsy, with a few errors coach Claude Julien said were more than understandable.

Paired with Johnny Boychuk, Krug jumped off the bench and immediately found himself in the thick of things. The Penguins were attacking in the Bruins’ zone, and the puck fell to Krug’s stick. He froze for a second, then made a failed attempt at a clearance.

“You can see him in the defensive zone trying to think about how to play in our system,’’ said Julien. “And sometimes, maybe there was a fraction of a second delay, which is totally normal. But once he had to do it, he went.’’

Krug seemed composed as he logged more than 18 minutes of ice time. When the Bruins were aggressively trying to make a third-period comeback, Krug was the only defenseman left behind the blue line and he intercepted a clearance that could’ve left the Penguins with a three-on-none.