Kessel, who was diagnosed with testicular cancer a month ago, played his first NHL game since having surgery Dec. 11 to remove one of his testicles.
"It felt good to play," Kessel said. "I was just excited to play tonight."
Senators scoring leader Dany Heatley tied it at 2 with his 26th goal at 5:43 of the third after Mike Comrie ruined Boston goalie Tim Thomas' shutout bid with his first Ottawa goal 2:22 into the period.
"There was a sense of confidence in the dressing room, even though we were down two goals after the first period," said Comrie, who was acquired from Phoenix in a trade Wednesday.
Heatley made a sensational play to set up the Senators captain's go-ahead goal as he swept the puck toward the slot while he was sliding headfirst along the ice on the left side.
Alfredsson spun around and beat Thomas to give Ottawa -- which fell behind 2-0 just over midway through the first -- its first lead.
"I didn't know he was there," Heatley said. "I just got bumped, and I fell and knocked the puck in front, and he was there."
Thomas, who made 31 saves, didn't feel that the Bruins sat on their two-goal lead.
"We didn't try to, that's the way it ended up working out," Thomas said. "It wasn't the game plan."
Kessel was given a warm reception from the crowd of 18,292 during a stoppage in play 9:49 into the game when "Welcome Back, Phil Kessel" was displayed over his image on Scotiabank Place's video scoreboard.
The 19-year-old rookie was also "welcomed back" moments later when he was on the receiving end of an open-ice check by Joe Corvo, and another late in the second from Comrie.
"He never said anything, and he finished the game," said Bruins coach Dave Lewis, who also noted that Kessel's quick return following surgery "says a lot about him."
While Kessel said he didn't notice the crowd's response, the checks didn't escape his attention.
"I felt those, but that's hockey," said Kessel, who shrugged off both hits. "It's no big deal. You just get back up and go play."
Comrie admired the youngster's attitude and enthusiasm for the game.
"I told him, 'I just tried to finish my check,' and he actually chuckled about it," Comrie said. "He's a hardworking kid and it's nice to see him on the ice."
Boston, which played without injured captain Zdeno Chara, lost for the first time in four meetings with the Senators this season.
The Bruins, who were also missing defenseman Brad Stuart, got off to a fast start, taking a 2-0 lead 11:37 in despite losing another defender to injury when Jason York left the game with a right knee injury during his fifth shift and did not return.
With Bruins defenseman Milan Jurcina off for holding, Sturm was awarded a short-handed penalty shot early in the first after he was hauled down from behind by defenseman Andrej Meszaros during a breakaway.
Sturm, who failed to score against Washington's Brent Johnson on Nov. 18 in Boston's only other penalty shot this season, deked Emery by stopping sharply as he drove the net before lifting a shot over the Ottawa goalie's right pad.
- Lewis used his time-out following Heatley's goal.
- Kessel has five goals and four assists in 28 games this season. He scored one goal during a two-game conditioning assignment with Providence of the AHL over the weekend.
- Sturm has two goals in four shootout attempts this season. He is 4-for-10 overall.
- Comrie has one goal and two assists in four games with Ottawa.