Before this season, Mazzulla had never finished higher than third in Coach of the Year voting.
He placed third after a 57-25 debut season and fourth after a 64-18 campaign in 2023-24. He received zero votes last season.
The award came after a year of lowered expectations following Tatum's torn Achilles. Jaylen Brown stepped up, averaging a career-high 28.7 points per game.
Although Bickerstaff was considered the frontrunner in April, media sentiment shifted toward Mazzulla before the announcement.
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Mazzulla has a history of downplaying individual honors. He once told guard Derrick White that "nobody cares" after receiving a Coach of the Month award.