It was the second trade for a veteran blue-liner in two weeks for Montreal, which dealt forward Travis Moen to the Dallas Stars for defenceman Sergei Gonchar on Nov. 11.
"Allen's a big guy with a lot of character who played for one the best Western Conference teams," Canadiens general manager Marc Bergevin said before Montreal's home game against the St. Louis Blues. "I can't see why he wouldn't be able to help us here in Montreal."
Like the Gonchar deal, the Canadiens picked up a player whose contract expires at the end of this season for one with more than one season remaining on his deal. The 34-year-old Allen earns US$3.5 million this season while Bourque is signed through next season at a cap hit of $3.3 million.
"One of our biggest strong suits is our depth on defence," said Bergevin, who wouldn't reveal what the trade meant for young defencemen Nathan Beaulieu and Jarred Tinordi. "Defencemen are the players who get hurt the most often. We've been lucky until now, but having greater depth on defence is a luxury."
Allen had one assist in six games for Anaheim this season. The Kingston, Ont. native missed 14 games with a lower-body injury.
He was picked fourth overall by Vancouver in 1998. In 716 career regular season games with Vancouver, Florida, Carolina and Anaheim, he has 29 goals and 106 assists.
"He's a big kid, a stay-at-home defenceman," said Bergevin. "He's not going to run numbers. Penalty killing probably, play against the other teams' bigger players. He brings character and grit and size on the backend."
Bourque cleared waivers and was assigned to Montreal's AHL affiliate, Hamilton Bulldogs, on Nov. 11 after a sluggish start to the NHL season with two assists in 13 games.
Bergevin said Bourque, who joined the Canadiens on Jan. 12, 2012 in a trade from the Calgary Flames, wasn't too distraught about leaving the organization.
"He's going from Hamilton to Anaheim," said Bergevin. "No, he wasn't upset. Rene understood. He was ready to leave, ready to turn the page. I won't be surprised if he does well with the Ducks. That's a really good hockey team. He can produce, and I hope he does."