While Kelly enjoyed his role in helping to craft Boston's young prospects, he missed that daily grind and the feeling of contributing to a team's success on a consistent basis. So, when a position on Cassidy's staff opened after the departure of Jay Pandolfo to Boston University earlier this summer, Kelly jumped at the chance to get back on an NHL bench and join fellow assistants Kevin Dean, Joe Sacco, and Bob Essensa.
"You're kind of in the fight with the players, in terms of you're there day in and day out," said Kelly, who was an interim assistant coach with the Ottawa Senators for roughly 20 games at the end of the 2018-19 season. "You can see the small differences that potentially help younger players grow their game. With development, there's that growth but you're not with them day in and day out.
"They're at their club team, they're in college, they're in junior. You're seeing them periodically, so you don't see the growth, I found, as much. When you're there every day, you can really help the players and really break it down a little bit more."
Kelly also arrives with an already stout understanding of how Cassidy and his staff operate on a daily basis having spent the last two years with the club's front office. The Ontario native also played for Cassidy as a 20-year-old with the AHL's Grand Rapids Griffins.
"I was fortunate enough to play for Bruce my first pro season," said Kelly. "And watching his teams from afar, he's an excellent coach. Obviously, Hockey Canada thought so as well [by naming him to the Olympics staff]. His in-game adjustments, the way he plays on the offensive side of the puck as well as the defensive side…you've seen players thrive in his system stats-wise…I'm super excited to learn from all the coaches. They all do a fantastic job."
Kelly is not quite sure how his daily duties will be assigned just yet, but he expects that much of his focus will be centered on "the defensive side of the puck." The 40-year-old, who played 288 games with Boston from 2011-16, will also be in a unique position to guide players - particularly the B's crop of new free agents, including his former Senators teammate Nick Foligno - through what it means to don the Spoked-B, given that he was part of the leadership core who helped create the culture that still exists within the Bruins dressing room.
"You hear about the Bruins culture throughout the league…when you go other places, other organizations, they talk about it and it's a thing throughout the league, how well the Bruins treat their players and the way the players [conduct themselves] - it's a culture thing," said Kelly.
"That competitiveness that their top-end players bring each and every day…I think Boston's extremely fortunate to have the players that they have leading, not only the young players, but the new players that come in. I was one of those players. I came in and was fortunate enough to be part of that team that won.
"To see how hard they were working right after they won the Stanley Cup in the summer to come into camp the next season, I think it helped me maybe carve out a few extra years in the NHL because of the work ethic that they were able to instill in me as an older player. To be able to come back and try to help with that on the other side is great."