The Senators captain has been the most dynamic player on the ice during the series, and his world-class plays -- like the perfect cross-ice pass to wide-open teammate Derick Brassard for the one-timer that tied Game 2 -- are the major reason Ottawa is up 2-1 heading into Game 4 on Wednesday at TD Garden in Boston.
Now the Bruins need their best player -- winger Brad Marchand, who led the team with 39 goals and 85 points during the regular season -- to be similar force if they are to get back into the series.
Ottawa has done a terrific job frustrating Marchand and keeping him off the score sheet. The Senators held the winger to just one shot on goal in Game 3, his second in a row without a point, and goaded him into a penalty.
"Well, clearly, he took a penalty [in Game 3], probably out of frustration. But listen, Brad Marchand was, what, the fifth leading scorer in the National Hockey League this year? So he's going to get keyed on," said Bruins coach Bruce Cassidy after Game 3. "Part of the process for him becoming an elite player is to play through that, [and] take advantage of the opportunities."
Marchand, who was the hero in the series opener, was on the ice for Ottawa's winning goal in Game 3, and also had a team-high three missed shots and two giveaways during 21:22 of ice time.
"He scored a big goal for us in Game 1 and was the difference in that game," said Cassidy. "We'll talk to him about it, but at the end of the day that's what happens when you're an elite player. You're going to get marked. You've got to find your way through the checking part of the game."
Being a marked man is nothing new for Marchand. Teams routinely target him, both because of his considerable offensive talents and his tendency to let opponents get under his skin. But Cassidy believes that the Bruins can help take the pressure off Marchand by better containing Karlsson.
"They've got a number of guys; they're a good defensive team," said Cassidy. "It's not like one guy's all over him. He's just one of the guys you circle on the board. We've got to do a better job with Karlsson. He's a guy that we've got to do a better job of chasing the puck out of his hands."
Marchand has been a game-changer on hockey's biggest stage before. As a rookie in 2011, he helped the Bruins to a Stanley Cup with timely goals and playmaking synergy with linemate Patrice Bergeron. That spring, Marchand had 11 goals and eight assists for 19 points in 25 playoff games. His two goals and one assist keyed Boston's Game 7 victory over the Vancouver Canucks.
In his 44 playoff games since, however, Marchand has scored only six goals. His goal in Game 1 of this series proved to be the winner, but he has been held scoreless since -- and had only one shot on net in Game 3.
"You've got to push through it," Marchand said. "Obviously, I have to be better. It's not my best so far. Guys have done a really good job of stepping up every night. We have four lines going, and that's what we need."
What the Bruins need most is for Marchand to return to form and start scoring the big goal he's known for, like the clincher he netted with less than three minutes to play in Game 1 last Wednesday.
"He's been playing well, but obviously it's much harder in the playoffs," said Bruins' David Krejci of Marchand. "He's one of the better players in the league, so they'll be playing him hard. You need your linemates, your teammates to help you out a little bit."
It doesn't help that Marchand is facing one of the best defensemen in the world. It almost seems like Karlsson knows exactly where Marchand is at all times.
"He's definitely getting chances," Karlsson said. "He's a good player. He's been a terrific player all year for them, and he's someone you have to pay extra attention to because he's still going to create his chances. You've got to try to make it hard for him and make him work for that little extra."
Senators goalie Craig Anderson said the fact that he hasn't seen as much of the 5-foot-9 Marchand in the dirty areas in front of the net is a testament to Karlsson and the other members of Ottawa's defense.
"We've got big, strong defensemen, and he's a strong little guy," Anderson said. "He likes to get to those hard areas, and we're making it tough for him."
But Anderson knows that Ottawa can't let its guard down against Marchand.
"He's a great player," Anderson said. "He's going to make good plays. You've got to respect him and keep an extra eye on him and know where he's at. [We can't] make it easy for him."
His coach has confidence that Marchand will find to his winning form.
"Marchy can create a little more out there," Cassidy said. "And he will."