Subban was given a five-minute major and a game misconduct for slashing Ottawa's Mark Stone at 8:23 of the second period.
With the puck near Carey Price's net, Montreal's star defenceman lifted his stick in the air and brought it down on Stone's right wrist.
The 22-year-old Stone fell to the ice in agony and then skated off to the dressing room. The Senators scored twice and gave up a short-handed goal to Lars Eller on the ensuing power play.
Stone was back at the Senators' bench roughly 10 minutes after the incident, left the game again in the third period, and returned once more.