''The reason why, I'm not exactly sure. But he did give some reasons regarding family,'' Chiarelli told reporters in a conference call originally scheduled to discuss the contract extension of forward Daniel Paille. ''As of right now, I'm operating under the premise that he will take a year off.''
A late bloomer who played in Finland before finally breaking into an NHL lineup at the age of 32, Thomas emerged as one of the league's top goalies when he won the Vezina Trophy as the league's top goalie in 2009. He won it again along with the playoff MVP in 2011 while leading Boston to the Stanley Cup championship.
But he is also an iconoclast who was known to wander far from the crease in games and occasionally leave his comfort zone off the ice as well. When the Bruins met President Barack Obama to celebrate their NHL title, Thomas skipped the White House visit and issued a political diatribe on his Facebook page as explanation.
Chiarelli said Thomas appeared tired after the championship season, when he played in 82 games, including every minute of the longest postseason in Bruins history. He got very little time off down the stretch this year after backup Tuukka Raskwas injured at the beginning of March.
''After we won the Cup, he was tired,'' Chiarelli said. ''With all the stuff that's been going on the last two years, with the winning and the appearances, I think he's worn down.''
Thomas, 38, could not immediately be reached for comment. Chiarelli said it was likely Thomas would post something on his Facebook page on Friday.
Thomas has a 196-121-45 record in a little more than seven NHL seasons, with a 2.48 goals-against average and 31 career shutouts. He is 29-21 in the playoffs, with a 2.07 goals-against average and six shutouts - four of them in the Stanley Cup season, when he won the Conn Smythe Trophy as the playoff MVP.
''I'm disappointed, but these things happen. You've got to deal with them,'' Chiarelli said. ''That was a strength of our team.''
Without Thomas, the Bruins would be left with Rask and Anton Khudobin as their top two goaltenders for next season. Rask has long been projected as a rising star; he supplanted Thomas as the starter in 2009-10, but Thomas regained his position the next year and remained there.
''We've got two very capable goalies,'' Chiarelli said. ''I'll be more than satisfied if that's what we have to go with.''
Logistically, Chiarelli said, if Thomas decides to take the year off, the team would have to suspend him to avoid paying him. Although the Bruins would have the right to toll his contract, extending it for another year at the same terms, they would have to take a salary cap hit of $5 million this year and again in 2013-14, the GM said.
Chiarelli said Thomas expressed an interest in playing in the 2014 Olympics, which will be two months before he turns 40. But he is not certain to be picked for the U.S. team over Ryan Miller, Jonathan Quick, Jimmy Howard or another young American goaltender who might have emerged by then.
Thomas was a member of the U.S. team in Vancouver, but he did not play on the team that won the silver medal.
Practically speaking, a year off for Thomas could translate into an early retirement.
''It would be hard for a 38-year-old to come back'' after a year off, Chiarelli said. ''I would think that.''