Sam Cassell’s been the heir apparent for multiple head coaching jobs over 15 years. It’s an inescapable cycle for one of the NBA’s most recognizable names. Next season, Cassell will join the Celtics bench as an assistant to 34-year-old Boston head coach Joe Mazzulla. That feels backwards. If these two individuals walked into the Celtics HQ for a job interview, Cassell would be spoon feeding Mazzulla some of his extensive NBA wisdom. Midway through, Mazzulla might think he’s been propped into an episode of Undercover Boss.

At 51, Cassell hasn’t been able to clear that final step to becoming a head coach. He’s won three NBA titles, mentored numerous All-Stars and he’s had the biggest cajones in the league for decades. His moxy and attitude is exactly what this roster needed. As the only representative of Boston’s revered 2008 squad who is a reputable coaching candidate, Cassell is a no-brainer to be Mazzulla’s top assistant or associate head coach.

Cassell has succeeded everywhere he’s been

In his first head coaching stop, the Washington Wizards, Cassell was assigned with developing No. 1 overall pick John Wall. How much of an impact a coach can have on the development of a point guard is as much of an inexact science as coaching trees being a reliable method of finding head coaches. Elite coaching isn’t a skill learned through osmosis. However, Wall swears by him. In Philly, he played a significant role in the development of Tyrese Maxey.

At each stop, Cassell has been passed over for the head job. But for the first time in his career. Cassell is on staff as the senior coach. Brad Stevens is committed to giving Mazzulla at least one offseason to complete his metamorphosis into the type of coach a championship contender needs. Cassell is built from the same no-nonsense stock as Ime Udoka. As a player and assistant, Cassell has been a respected voice on multiple benches from Washington to the L.A. B-Team to Philly.

It’s a bleak statement on the NBA’s hiring managers that we live in a timeline where Steve Nash or Chauncey Billips have been head coaches before Cassell. That’s not a knock on Nash. But Cassell has been plying his craft on coaching staffs since Nash was swerving around pick-and-rolls in Phoenix.

And his presence will loom large every time Joe Mazzulla coughs up a losing streak or botches a late game adjustment. Mazzulla was named interim head coach in late September and never had an opportunity to replace the valued assistants from Boston’s Finals run. Will Hardy was plucked away to lead Utah’s rebuild and Damon Stoudemire returned to college hoops after being named Georgia Tech’s head coach in the middle of the season.

Will Cassell move into the top job?

Aside from a veteran Coach of the Year candidate on the open market, Cassell is the top assistant coach whose resume is buzzing in the coaching carousel every season. Cassell was even a candidate for the open job that Stevens eventually awarded to Udoka. On the bench, Cassell is as much an energy guy, grinder and cohesive locker room personality as he was during his playing career.

During a pivotal season which could be Jaylen Brown’s last in Boston before he tests unrestricted free agency, and one in which they’re simultaneously be expecting to win, not just compete for a title, urgency is paramount. Mazzulla is Stevens’ guy, but the NBA is a ruthless business. We’ve seen this play out before.

In Cleveland, David Blatt was cast aside for assistant Tyronn Lue, a year after leading the Cavaliers to the Eastern Conference Finals. Four months later, the Cavaliers were celebrating an improbable 3-1 comeback in the NBA Finals against the 73-win Warriors. In 1977, Billy Cunningham replaced Gene Shue after the latter led Philadelphia to a runner-up finish in the 77 Finals. Cunningham stuck around long enough to become an NBA champion on those Knicks. Pat RIley ascended from assistant on Paul Westhead’s staff to head coach six games into the ‘82 season. Jeff Van Gundy became a Knicks legend only after Don Nelson was chucked into the Hudson River after 69 games.

Mazzulla is the head of the Celtics snake, but Cassell will be the chief of staff. The Celtics needed a backstop on their coaching staff in case Mazzulla goes catatonic on the bench — and Cassell is that guy.

Follow DJ Dunson on Twitter: @cerebralsportex