Bruins coach Claude Julien did not want to blame the loss on the Senators' defensive scheme, but rather his team's mistakes with the puck.
"Tonight was an issue because we weren't good," said Julien, whose team has now lost two in a row. "As much as I'd like to give them credit for the neutral zone, I have to really blame ourselves for how poor we were with the puck.
"We've been a better team than that. It's disappointing that tonight, in an important game like this, we came out with probably one of our worst efforts in probably a month.
"I didn't like our game tonight, decision making, puck management. I would say there were a lot of no-shows as well. Those are things that can't happen in these kinds of games."
Leading, 1-0, almost midway through the second, the Bruins began to see their mistakes prove costly, when Mark Stone slipped in all alone on Tuukka Rask and buried one to tie the game at 13:23.
Boston escaped the second without allowing any further damage, but the Senators came out strong in the third to take control. After a turnover in the slot, the puck trickled out to Chris Wideman, who fired a shot from the point that deflected off Dominic Moore and past Rask to give Ottawa a 2-1 lead at 13:32.
Just over three minutes later, the Senators made it a 3-1 lead. Following another turnover, this one just inside the Ottawa blue line, the Senators turned up ice with pace, resulting in Kyle Turris beating Rask at 10:16.
"I always try to be ready for the turnovers," said Rask, who made 23 saves. "But today we gave the puck away a lot. I think we got a little spread out at times, too.
"We turned it over and there were no numbers coming back and caused scoring chances."
Now, the Bruins must turn their focus to Friday night, when they cap off a back-to-back by hosting the Calgary Flames at TD Garden.
"We've just got to forget about this as quick as possible and get ready for tomorrow. That's the only thing we can do," said Rask. "A lot of times when you play and win you want to carry it over, but we definitely don't want to carry this over to tomorrow."