General manager Don Sweeney said Rask will back up Linus Ullmark for Wednesday night's game against the rival Montreal Canadiens, with 23-year-old rookie Jeremy Swayman headed to Providence of the AHL. The Bruins are also scheduled to play on Thursday, against the Philadelphia Flyers, but no decision has been made, Sweeney said.
Rask, 34, was a free agent who remained unsigned while he recovered from an operation to repair a torn labrum in his hip. He agreed to a prorated $1 million, one-year deal that will pay him $545,000.
Sweeney thanked Rask and his agent, Markus Lehto, for the team-friendly deal.
"He just wanted to be part of a group that he spent his entire career with," Sweeney said. "They're not blind to the salary cap challenges we all face. I appreciate them working with us on that."
Rask said at the end of last season that he did not want to play for any team other than Boston.
"This is our home," he said. "At this point of my life and my career, I don't see any reason to go anywhere else, especially with the health I'm looking at now and a recovery time of five or six months. Hopefully it works out that I recover well and we can talk about contracts when the time is right for that."
Sweeney said that Swayman, who is 8-6 with a 2.26 goals-against average, was disappointed but not surprised by the demotion. Ullmark, 28, a veteran who is in the first season of a four-year, $20 million contract, is not eligible to be sent to the minors.
"Jeremy took it in stride as a professional," Sweeney told reporters. "He's extremely motivated to be a top-flight goaltender. He knows he's a big part of our current team, as well as moving forward. He's got a hell of a long career in front of him."
Rask played in only 24 games last season as the team managed his workload while he fought hip and back injuries. After a second-round loss to the New York Islanders, he revealed that he needed hip surgery.
Rask began skating with the Bruins again this winter and signed a professional tryout agreement with Providence last week, but the AHL team had two games scrubbed because of COVID-19 protocols. Its next scheduled game is Friday night.
"I feel great," Rask said last week during a video call with reporters. "The biggest issue for me was the catching of the joint and the pain that created. So that all is gone. ... I don't have to think about it locking up on me again and creating that pain, so I feel great."
Rask said he never really considered retiring this offseason. The lure of being able to make one more run at a Stanley Cup and finish out his career alongside stars Patrice Bergeron and Brad Marchand played a factor in his decision to attempt a comeback.
"That's why I never really, in my head, wanted to flirt with opportunity to go somewhere else," Rask said. "For us as players, when you have a team like the Bruins, basically a bunch of us have grown up together. So, you kind of feel that brotherhood. You don't want to leave guys on bad terms.
"I just wanted to come back and maybe be helpful and try to finish it out with a bunch of those guys I've played with my whole career."
The 2014 Vezina Trophy winner who was also a finalist in 2020, Rask led the Bruins to the Stanley Cup Final in 2013 and again in 2019. He is the franchise's career leader with 306 regular-season wins (57 more in the playoffs), 560 regular-season games (104 in the playoffs) and a .921 save percentage.
In all, he is 306-163-66 with a 2.27 goals-against average and 52 shutouts. Last season, he was 15-5-2 with three shutouts, a 2.28 goals-against average and a .913 save percentage.