A year after trading Kevin Garnett to the Brooklyn Nets, the Boston Celtics made Marcus Smart the sixth overall pick of the 2014 NBA draft. Since then, Smart has become the Celtics’ unquestioned defensive leader and is currently the longest-tenured Celtic heading into the 2021-22 season.Marcus Smart is one of those players whose impact is not oftentimes reflected on the stat sheet. He routinely makes game-changing plays that are not measured by numbers. A closer look at his performance, however, would also confirm what the eye is appraising.ℤ𝕒𝕔𝕙 Davis@ANTIUSOfficialTatum's 50 was excellent but we don't win this game without Marcus Smart. He always plays so good in the playoffs. 23 points on 8-11 shooting (and excellent shot selection) with 6 assists... And that All-NBA defense. #Celtics12:41 PM · May 29, 202181Tatum's 50 was excellent but we don't win this game without Marcus Smart. He always plays so good in the playoffs. 23 points on 8-11 shooting (and excellent shot selection) with 6 assists... And that All-NBA defense. #Celtics https://t.co/wV8dmvzgXFSmart is a two-time All-NBA Defense and the first Boston Celtics player to accomplish the feat since the legendary Big Ticket was still guarding the paint with a snarl for the Cs. Smart also had career highs in scoring (13.1 PPG), assists (5.7 APG), and minutes (32.9 MPG) last season.With more things to check off his to-do list for the Celtics this year, he could be an even bigger difference-maker.Why Marcus Smart will be a game-changer for the Boston Celtics in the upcoming seasonBoston Celtics Media Day#3 Becoming the Boston Celtics’ lynchpin on defense againBy his lofty standards and admission, Marcus Smart significantly regressed in his defensive performance. It’s not a coincidence that the Cs slipped out of the top 10 in defensive ratings when Smart was not at his best.PlayerSeasonDefensive RatingMarcus Smart2018-19107.12019-20107.22020-21113.2TeamSeasonDefensive RatingBoston Celtics2018-19107.82019-201072020-21112.5Nobody on the Boston Celtics’ roster changes the game the way Smart does when he rolls up his sleeves to guard the best players on the opposing team. If he gets more offensive duties this season as the starting point guard, it should not make much of a difference on the defensive side.Brad Stevens has tinkered with the roster to make the Celtics more balanced. The return of Al Horford and the addition of Josh Richardson could be underrated signings that’ll shore up the defense. With Smart leading the defense, this unit is expected to go back up in defensive ratings.He’s practically been the point guard because Kyrie Irving and Kemba Walker, respectively, have been in and out of the lineup for the past few years because of injury.Ime Udoka’s defensive mindset will only highlight Marcus Smart’s importance this season.