The 21-year-old defenceman assisted on a Max Pacioretty goal and was plus-2 on the night in the Montreal Canadiens' 4-0 victory over the Boston Bruins in Game 6 of their best-of-seven Eastern Conference semifinal.
Beaulieu is expected back in the lineup for the decisive Game 7 on Wednesday night in Boston.
"What I like about Nathan is his speed and his poise with the puck, his ability to move the puck," Therrien said Tuesday. "We understand we put him in a tough position, his first game in the NHL in the playoffs, but it's a process.
"He's been with us at times this season and he's working really hard with the black aces. For the last week, we've been thinking about when will be the right time. We didn't have any fear to put him out there because we know the kid's got character and he's got speed and moves the puck and is capable to compete."
Beaulieu is the third player to skate in the left defence position on the third pairing, beside veteran Mike Weaver, in these playoffs.
Francis Bouillon played in a first-round sweep of Tampa Bay and the first two games of the Boston Series. Then they opted for greater size in Douglas Murray for three games before going with the skilled but untested rookie.
Beaulieu, drafted 17th overall in 2011, played 17 games after three call-ups from AHL Hamilton this season and had two assists. In 23 regular season NHL games over the past two campaigns, he has no goals and four assists.
The six-foot-one 185-pound native of Strathroy, Ont., was on the ice when Lars Eller opened the scoring 2:11 into Game 6 and then made a stretch pass that sent Pacioretty in for the Canadiens' second goal in the middle period.
"I got a member's bounce there and he made a nice play to slide it five-hole," Beaulieu said of the goal. "When you come here as a black ace, you've got to stay ready.
"You have to come here with expectations that anything can happen. The best way to put it is I was overwhelmed, very happy with the opportunity. I just tried to take full advantage of it."
Beaulieu is no stranger to playoff hockey. He was the top point man for the Saint John Sea Dogs team that won the MasterCard Memorial Cup in 2011. His head coach that season was Gerard Gallant, now a Canadiens assistant coach.
"In junior I played a lot of championship games, but this is for sure the biggest game I've ever played in my life," he said. "It was a lot of fun."
There were reports that Beaulieu was not happy when he was sent back to Hamilton in February after an 11-game stint in which he played solid two-way hockey. But Canadiens management preferred to take their time with what they see as a top defence prospect.
"I trust management's decision," said Beaulieu. "If they don't think I'm ready, I believe in them. I was very grateful for the opportunity and tried to make the most of it."