There are many reasons as to why the Bruins find themselves down two games to one entering Wednesday’s pivotal Game 4 at TD Garden. Untimely penalties, long stretches of poor play and a depleted blue-line are just a few of those reasons.

Erik Karlsson is another.

The Senators’ captain has by far been the best player on the ice through three games and is arguably the biggest reason why Ottawa is now just two wins away from advancing to the second round of the Stanley Cup Playoffs.

Karlsson has always been known as one of the better offensive defensemen in the league, but his play in his own zone has always come into question.

The 26-year old Karlsson has done a tremendous job of rounding out his entire game of late and the new and improved Karlsson has been front and center for the Senators. The Bruins haven’t had issues putting the puck in the net with Karlsson on the ice as four of the eight Bruins’ goals in the series have come with number 65 on the ice, but it’s what he’s done with the puck that has given the Bruins fit.

With the Bruins holding onto a one-goal lead in the third period of Game 2, Karlsson created space for himself, took advantage of the created space and sent a laser of pass over to Derick Brassard for the game-tying goal. Ottawa would go on to win in overtime after erasing a two-goal third period deficit.

In Game 3, Karlsson found himself deep in his own zone with the puck. Instead of making the safe or easy pass out of the zone, Karlsson sent a beautiful lob pass out of the defensive zone, through the neutral zone and right onto the stick of Mike Hoffman who had just entered the Bruins zone. Hoffman raced in all alone and beat Bruins’ goalie Tuukka Rask with a dandy of a move for the Sens’ first goal. A goal that would not have happened if not for a perfect pass from Karlsson.

“I mean that’s Erik. Erik has done that all year long. What’s great is that, those are the highlights, but if you look at his defensive game, it’s been unbelievable all year long,” Senators head coach Guy Boucher said following Monday’s Game 3 victory. “And to me, he’s equally as good now on both sides of the ice.”

Anytime Karlsson touches the puck the Bruins need to be on high alert. His ability to turn nothing into something has burned the Bruins so far and an answer for Karlsson needs to be found.

Karlsson’s health was a hot topic entering the first round series as the captain missed five of Ottawa’s last seven regular-season games due to two separate lower-body injuries. With three games in the books, Karlsson has shown no signs of anything slowing down his game.

After seeing their once 3-0 lead disappear thanks to three unanswered second period Bruins goals, the second intermission was an important one for the Senators Monday night. In what could have been a somber and disappointing intermission for the road team, Karlsson did his job as captain to rally his troops. Once again, the Senators would go on to win in overtime.

“There was a sense of we didn’t feel like we let one get away,” Bobby Ryan said in regards to the feeling in the room after the Bruins tied the game. “Our captain [Karlsson] spoke. The right guy said the right message. We went out and we faced adversity whether it was on the ice or off the ice all year long with a lot of different situations and we responded very well and tonight wasn’t any different.”

After their win in Game 3, the Senators have regained home ice advantage – at least for the time being. A win Wednesday in Game 4 will give the Senators their first opportunity to end the Bruins season on Friday night back in Ottawa for Game 5. If that does end up being the situation, you better expect Karlsson to continue to be a reason for that.

With three one-goal games to start the series, this easily could be a 3-0 series in either direction. Karlsson knows the Senators haven’t been at their best and the dangerous defenseman hopes their best game is coming Wednesday night.

“[Monday] was a battle. It could have gone either way I think and we could have lost this game if we didn’t take care of the small things later on in the game and again we got to learn from that and we want to put together a better game you know than the first three that we have had here,” added Karlsson.

“I still think that we haven’t had our best game yet and hopefully we can have that on Wednesday.”

If the Bruins are to prevent the Senators from having their best game of the series, they need to start with shutting down Karlsson and going from there.