As reported by the Boston Globe's Fluto Shinzawa:

Patrice Bergeron did not return to Tuesday night's game after leaving halfway through the second period. Coach Claude Julien declined to disclose the nature of Bergeron’s injury. Bergeron was not in the dressing room after the 3-2 win over the Senators at TD Garden.

Bergeron was injured during a race for the puck with Colin Greening. When reaching for the puck, Greening clipped the left side of Bergeron’s head with his elbow. Bergeron dropped his stick, paused, then skated toward the bench.

Bergeron played only 9:06. He is the Bruins' best all-around forward and one of the three most important players on the team.

This was the only video of the collision that I could find: