Trouble was, the Bruins played that way for only one of last night’s three periods.

“The first two periods, we were forcing plays,’’ Patrice Bergeron said. “That’s a type of team you can’t do that against. If we would have played the same way all game that we did in the third, it would have been much better.’’

Ottawa scored a power-play goal in the first period. After holding serve in the second period, the Senators rode the perfect netminding of Robin Lehner in the third period to claim a 1-0 win over the Bruins last night before 17,565 at TD Garden.

The loss spoiled the first game for Brian Rolston and Mike Mottau since the Monday deal that brought them to Boston from the Islanders.

“Those first two periods were really painful to watch and to see,’’ coach Claude Julien said. “Our guys just didn’t have any legs. Our game was very, very slow. Even faceoffs. Everything. We just struggled in all areas.

“All of a sudden, we redeemed ourselves on faceoffs. Our game picked up in the third. We found our legs. Hopefully that’s a good sign for the next game.’’

The plucky Senators are just 1 point behind the Bruins in the race for the Northeast Division title. Ottawa has played four more games than the Bruins.

But last night’s game proved a point: The Senators aren’t going away.

“It’s up to us to just do the job,’’ Julien said. “This is an opportunity for us to have those games in hand and take advantage of it. I don’t think I’m really worried about them. Let’s worry about ourselves. Let’s just do the job. We should be fine.’’

The only goal of the night took place on a first-period power play. Tyler Seguin was called for an offensive-zone interference penalty. On the following power play, Ottawa defenseman Erik Karlsson walked the puck down the blue line into the middle of the ice, then spotted his opening.

As Daniel Alfredsson set up in his office in front of Tim Thomas, Karlsson hammered a slap shot through the screen. Thomas never saw the puck, which whistled between the bodies and into the net at 14:44. Karlsson, Ottawa’s flashy puck-moving defenseman, now has seven goals and 11 assists in his last eight games.

“I didn’t see it,’’ Thomas confirmed. “I barely saw it released. Where I saw it going was blocker side. I had to go behind Alfredsson to hopefully make the save to hope that it hits me. It actually did nick off me, I believe. Unless it was his stick. It felt the same way.’’

Had it not been for Thomas (37 saves), the Senators could have poured in more than just one goal. With each season, Thomas has settled down his game and developed into a technically efficient netminder. But because of the quality of Ottawa’s chances in the first two periods, Thomas played more like his previous version: an acrobatic whirlwind.

In the first, when Erik Condra had a net-front sniff, Thomas spun back in position to stop the shot. On several occasions, the Boston goalie was on his rear end when he stopped pucks.

“You need your goalie to be good on those nights to give your team a chance,’’ Julien said. “He gave us a chance. He was good.

“He made some saves Timmy-style. Bottom line, he was making saves and giving us an opportunity right until the end.’’

Despite the sluggishness of their first 40 minutes, the Bruins were still a mere strike away from tying the game. On the first shift of the third period, Seguin, Bergeron, and Brad Marchand - the team’s most effective line last night - had several chances in the opening minute.

Lehner (32 saves) turned the Bruins back.

Later in the third, Lehner used both his athleticism and his technique to foil the Bruins. When Benoit Pouliot tipped an Andrew Ference shot, Lehner recovered, reached back, and gloved the tip at 7:24.

When David Krejci ripped off a shot from the slot, Lehner was in the right spot to catch the puck and stop the action at 10:54.

When Milan Lucic and Dennis Seidenberg put back-to-back pucks on goal, Lehner stood tall to keep both shots out of his net.

The Bruins ripped 17 shots at Lehner in the third. The young netminder, called up only because starter Craig Anderson is out with a hand injury, responded to post his first career shutout.

“We had a lot of chances, especially early on in that first shift,’’ Bergeron said. “Our line got two great chances. We’ve got to find a way to put those in.

“Still, the other lines had some good chances. There was no traffic or a second rebound. He made some good saves.’’

Fluto Shinzawa can be reached at fshinzawa@globe.com. Follow him on Twitter @GlobeFluto.