During the third period, with the Bruins up 3-2, Krug skated from one side of the ice to the other and delivered a monster blow to Blues center Robert Thomas.
The hit sent the TD Garden crowd into a frenzy.
Krug had lost his helmet in a scuffle with David Perron, according to the Boston Herald.
“It’s part of the game. Momentum swings,” he said after the game, according to the newspaper. “It gives your team a boost of energy. If it pushes them back, I don’t know what they were feeling on their bench, but if it pushes them back and catches them off guard, then great for our team. But I think it gave our team energy, and that’s all you’re trying to do out there is make little plays that push your team in the right direction, and that was one of them.”
Minutes later, Bruins star Brad Marchand put in an empty-net goal to give the Bruins a 4-2 victory over the Blues.
Boston has a 1-0 advantage in the series. Game 2 will be played Wednesday at 8 p.m.