Krug was undersized for an NHL defenseman (5-foot-9, 186 pounds), which may have played a part in him going undrafted. But after making his name known in the 2013 Stanley Cup Playoffs, he become a staple of the Boston Bruins defense corps.

Born in Livonia, Michigan, Krug advanced through the Belle Tire hockey system and played one season for Indiana of the United States Hockey League before enrolling at Michigan State University.

In three seasons with the Spartans, Krug had 83 points (26 goals, 57 assists) in 114 games. As a junior in 2011-12, he had 34 points, including 12 goals, and was named CCHA Player of the Year, was named to the NCAA West First All-American Team, and was a finalist for the Hobey Baker Award, given to the nation's top college player.

As a result, Krug was a highly sought-after free agent and ultimately signed with the Bruins on March 25, 2012.

He jumped in right away for Boston, making his NHL debut against the Pittsburgh Penguins on April 3, 2012. Two nights later, he had his first NHL point, an assist, in a 3-1 victory against the Ottawa Senators.

Except for one game, he played the 2012-13 regular season for Providence of the American Hockey League, where he had 45 points (13 goals, 32 assists) in 63 games.

Then came the 2013 postseason.

Short-handed on defense because of injury, Krug was asked to fill a bigger role, and he took full advantage of the opportunity. Krug scored four goals, including three on the power play, added two assists, was plus-5 and averaged 15:49 of ice time in 15 games to help Boston advance to the Stanley Cup Final, where it lost in six games to the Chicago Blackhawks.

Undersized nor not, the Bruins were sold.

Any worries about Krug's size being a durability issue were put to rest during his rookie season in 2013-14, when he had 40 points (14 goals, 26 assists) in 79 games. In the 2014 playoffs, he had 10 points (two goals, eight assists) in 12 games.

Following Krug's third season, when he had 44 points (four goals, 40 assists) in 81 games, the Bruins signed him to a four-year, $21 million contract, one day before he could have become a restricted free agent. He rewarded Boston by scoring at least 51 points in each of the following three seasons, including an NHL career-high 59 points (14 goals, 45 assists) in 2017-18.

Krug would again shine during the 2019 playoffs, when he had 18 points (two goals, 16 assists) in 24 games to help the Bruins advance to the Cup Final. They fell just short again, losing to the St. Louis Blues in seven games.

After playing his first nine NHL seasons for the Bruins, Krug signed a seven-year, $45.5 million contract with the Blues on Oct. 9, 2020. He had five assists in a 7-2 win against the Montreal Canadiens on Feb. 11, 2024, to set the Blues' single-game records for assists and points by a defenseman. He is one of six skaters in Blues history with five assists in one game, joining Robert Thomas, Dallas Drake, Adam Oates, Bernie Federko and Brian Sutter. Krug was also the third player for St. Louis since 2003 with a five-point game, joining Thomas and Vladimir Tarasenko.