Grzelcyk was born to play for the Boston Bruins. The native of Charlestown, Massachusetts, attended Belmont Hill School before spending two seasons with the U.S. National Team Development Program, then came home and played four seasons at Boston University, serving as captain in the final two. If that's not enough, his father and brother each work at TD Garden.

The Bruins selected Grzelcyk in the third round (No. 85) in the 2012 NHL Draft and were content to keep an eye on him while he matured in college, where he had 93 points (26 goals, 69 assists) in 124 games and scored the overtime goal that enabled BU to capture the 2015 Beanpot tournament.

Grzelcyk signed with the Bruins on April 1, 2016, and spent most of the 2016-17 season with Providence of the American Hockey League, finishing with 32 points (six goals, 26 assists) in 70 games. He also made his NHL debut with the Bruins on Dec. 14, 2016. After opening the 2017-18 season with Boston, he spent the next six weeks at Providence before being recalled. Grzelcyk earned his first NHL point on Nov. 22, 2017, and scored his first goal two days later.

At 5-foot-9 and 174 pounds, Grzelcyk isn't going to be a physical force. But he's finding a niche as a smart, puck-moving defenseman who doesn't get overwhelmed by bigger opponents and finds ways to spend as little time as possible in his own zone, earning a regular berth in Boston's six-man rotation.