Boston tied the game with a goal by Viktor Arvidsson at 14:16 of the first period and took a 2-1 lead when Michael Eyssimont scored at 14:55 of the opening frame via a goal off the rush.
Morgan Geekie gave the Bruins a 3-1 lead at 19:55 of the second period.
With the Avs' net empty, Lehkonen scored his second goal of the game and fourth of the season via a net-front deflection on MacKinnon's slap shot from the point at 19:40 of the third period.