Boychuk/By S. Bradley

This evening the Bruins found out how much more than just a game they lost Saturday night against the New York Rangers at TD Garden.

Defenseman Johnny Boychuk, who was slashed by Brandon Dubinsky in the first period of the Bruins’ 3-2 loss, has suffered a slight fracture of his left ulna (bone in the forearm), general manager Peter Chiarelli revealed tonight.

Boychuk is expected to miss approximately four weeks. Amazingly, he returned to play a couple shifts of that opening period before sitting out the second and third stanzas Saturday.

The Bruins did not immediately announce a roster replacement for Boychuk, and they don’t have to make a move. Seventh defenseman Adam McQuaid has been cooling his heels as a healthy scratch for the first games of the season and could step into the lineup. Boston has until its next game Thursday night at home against Toronto to assess the situation.