Back for the third and it was none other than Lehkonen who scored on his former team to get the hosts on the board.
The Finnish forward took a pass from countrymate Mikko Rantanen and put it past Allen on the power play.
Just like Monday in Arizona, Wednesday's contest necessitated overtime to find a winner.
Rantanen won it for the Avs at 1:51 in the extra period, moments after the visitors were stymied at the other end.
Allen finished his night with 34 saves.
The Canadiens will be back in action on Friday when they take on the Stars in Dallas. Note that the game's start time has been rescheduled to 8:00 p.m. ET.