Auld faced 37 shots in the loss as he stepped in for Anderson, who is out indefinitely after cutting his hand Wednesday. Auld is the likely candidate to fill in, and while he looked nervous to start the game, he got better as the night wore on.
"I felt good, but it wasn't enough," said Auld. "I think it's a good step. I think obviously I can just worry about myself and the way I play and the way I prepare.
"I felt good in the game and it was nice to see a lot of shots, it's probably the most I've had all year, but you also play to win and you want to win games against teams you're chasing."
Tim Thomas stopped 27 shots for the Bruins to extend his unbeaten streak at Scotiabank Place to nine games.
Karlsson opened the scoring at 1:10 of the first with his 14th goal of the season, but Boston countered with Bergeron's first of the game at 3:36 before Marchand put the Bruins up 2-1 with 3:47 to go with his 22nd goal of the season.
"We got the start we were looking for, but then they score pretty quick after I lose a battle in our end and then they get a rebound and score," said Alfredsson. "If we can hold on there, maybe we can get our legs going, but that kind of gave them some momentum."
Camper made it 3-1 for Boston with his first goal of the season at 3:57 of the second before Krejci put Boston up 4-1 on the power play at 10:24 of the third.
"I was really pleased with the way we played in the second," said Boston's Zdeno Chara. "Usually we don't have such a good second period, but we had a really strong one."
Ottawa made it interesting on Alfredsson's two goals. He scored with 3:45 remaining before registering his 22nd of the season with 2:08 to go. Jason Spezza picked up his 600th career point on Alfredsson's first goal.
"We put a good push in the third period, but obviously the first two periods we're not too pleased," said Alfredsson.
"Credit to them for playing real solid. I didn't think we moved the puck quick enough and therefore we couldn't get them back in their end and so we were forced to play a lot of time in our own end and it's tough to get goals that way."
Bergeron rounded out the scoring on an empty-netter with 19 seconds left.
"He made a really nice big hit, it was clean, and that's part of the game, but also part of the game is sticking up for teammates and doing what you have to do," said Chara. "He didn't need to fight me, but I appreciate it."
The two teams meet again Tuesday in Boston.
- The Senators were without Anderson, Cs Peter Regin (shoulder, out for season) and Jesse Winchester (concussion, indefinite). D Brian Lee and C Zenon Konopka were healthy scratches.
- The Bruins were without C Marc Savard (post-concussion syndrome, indefinite), RW Nathan Horton (concussion, indefinite), C Rich Peverley (knee, four-to-six weeks) and LW Shawn Thornton (sick, day-to-day). D Andrew Bodnarchuk was a healthy scratch.