[8] He later played for the Belle Tire midget program in Michigan, and Krug skated in 59 regular season games for the Indiana Ice of the United States Hockey League (USHL), notching 47 points.

[citation needed] He was a member of the USHL All-Rookie Team and set the Indiana Ice record for points in a season by a defenseman in 2008–09.

[citation needed] Krug arrived at Michigan State University in the fall of 2009, and was initially slotted as a fifth or sixth defenseman on the roster.

Krug quickly gained a reputation as one of the CCHA's most offensive performers,[11] and in advance of his sophomore season, was elected captain of the Spartans.

[12] As the result of his accomplishments with Michigan State, Krug was among collegiate hockey's most sought-after free agents when he signed with the Boston Bruins in 2012.

[citation needed] Krug was frequently mentioned during the 2013–14 season as a strong contender for the Calder Memorial Trophy, awarded to the NHL's rookie of the year.

[25] While playing in the 2018 Stanley Cup playoffs, Krug injured his left ankle during Game 4 against the Tampa Bay Lightning and was later ruled out for the rest of the series.

[27] On March 27, 2019, in a game against the New York Rangers, Krug became the all-time points leader by an American-born player in Boston Bruins history.

[citation needed] On October 9, 2020, the opening day of free agency, Krug left the Bruins after nine seasons and signed a seven-year, $45.5 million contract with the St. Louis Blues.

[33] Torey admired the play of former Detroit Red Wings forward Pavel Datsyuk because "he's the most well-rounded player on the ice," and models his game after former teammate Zdeno Chára.

[38] Krug was forced to switch programs again after signing with Boston due to the mandatory courses in Law not being available online, leading him towards Political Science.