On June 15, 2011, Krejci got what he'd come to the NHL for.

Selected by the Bruins in the second round (No. 63) in the 2004 NHL Draft out of the HC Kladno junior program in the Czech Republic, Krejci joined Gatineau of the Quebec Major Junior Hockey League, where he 144 points (49 goals, 95 assists) in 117 games over two seasons.

Midway through his second season with Providence of the American Hockey League in 2007-08, Krejci was recalled for good and scored 27 points (six goals, 21 assists) in 56 games.

But it was the following season when he burst onto the scene, getting 73 points, including 51 assists, in 82 games. He also led the League with a plus-37 rating.

Two seasons later, Krejci would be a big reason the Bruins ended their 38-season championship drought.

In the 2011 Stanley Cup Playoffs, Krejci led the League with 23 points (12 goals, 11 assists) in 25 games, including scoring a hat trick in Game 6 of the Eastern Conference Final against the Tampa Bay Lightning, becoming the first Bruins player since Cam Neely (1991) to score three goals in a playoff game.

In the Cup Final against the Vancouver Canucks, he had six points (two goals, four assists) in seven games.

Although his career has been interrupted at times by injury, including a concussion in January 2007, a dislocated wrist during the 2010 playoffs, and a partial tear of the MCL in his left knee in February 2015, Krejci has proven to be a consistent forward for Boston, and after he had 69 points (19 goals, 50 assists) in 80 games in 2013-14, the Bruins signed him to a six-year contract extension on Sept. 4, 2014.

In 2018-19, Krejci tied his NHL career high with 73 points (20 goals, 53 assists) in 81 regular-season games, and he had 16 points (four goals, 12 assists) in 24 playoff games to help Boston reach the Cup Final again before losing to the St. Louis Blues in seven games.

Krejci signed a one-year, $1 million contract with the Bruins on Aug. 8, 2022. He returned to the NHL after playing the previous season in his native Czech Republic, where he had 46 points (20 goals, 26 assists) in 51 regular-season games and five points (three goals, two assists) in five playoff games for Olomouc in Czech Extraliga, the highest professional league in the country. He had three assists in his 1,000th NHL game, a 6-0 win against the Philadelphia Flyers 6-0 in Boston on Jan. 16, 2023, and had his 11th and 12th assists in a Game 7 to pass Doug Gilmour (10 assists) and Jaromir Jagr (10) for the most in NHL history, each coming in a 4-3 overtime loss to the Florida Panthers in the deciding game of the 2023 Eastern Conference First Round.