Three years ago, Joe Mazzulla was the head coach of Division-II school Fairmont State. Just over three months ago, he was coming off his third year as a Celtics assistant coach, still seemingly years away from his first shot as a head coach at the professional level.

Now he’s the favorite to win NBA Coach of the Year. And at this rate, it’ll take a strong push to dethrone him.

The betting markets have barely been able to keep up with the stunning rise for Boston’s interim head coach, who was hardly on anybody’s radar over the summer. Ironically, former Celtics coach Ime Udoka was the consensus favorite to win the award as recently as September after leading his team to the NBA Finals a year ago.

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Then the team suspended Udoka for the season and turned to the 34-year-old Mazzulla, whose only NBA experience to that point came in the last three seasons on Boston’s bench. At first, oddsmakers didn’t even list Mazzulla in the Coach of the Year market, instead opting for the Timberwolves’ Chris Finch and Grizzlies’ Taylor Jenkins atop the board.

Finally, oddsmakers installed Mazzulla among the preseason favorites by October, pricing him at 12/1 at BetMGM among the likes of Finch and Jenkins. It didn’t take long for Mazzulla to seize the top spot on the board after his Celtics won 21 of their first 26 games — with three of those losses coming in overtime.

That scintillating start earned Coach of the Month honors for Mazzulla, who was favored for this award by November and nearly an odds-on favorite by December. He remains the top choice in the betting market, sitting at +260 at DraftKings. But he’s got competition.

Nets coach Jacque Vaughn — another coach who started with an interim tag — was similarly off the board entering the season, when Steve Nash remained at the helm. Vaughn took over as the permanent head coach in November after Nash’s unceremonious exit and was still a long shot for this award as recently as last week, when he was dealing at 14/1 at BetMGM.

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Then the wins started piling up for Brooklyn, which had rattled off an NBA-best 12 straight wins entering Wednesday’s tilt with the underdog Bulls. That midseason surge has Vaughn (+280) nipping on Mazzulla’s heels in the Coach of the Year market and makes for a compelling contrast between the top two candidates, both thrust into unenviable positions with loads of talent at their disposal.

Do you favor the first-time candidate whose team seems likely to finish with the best record in the NBA, thanks largely to his steady hand after shocking offseason adversity? Or do you side with the longtime assistant and former head coach whose second stint stabilized perhaps the biggest in-season circus that we’ve ever seen?

They aren’t alone in this race, either. Pelicans coach Willie Green (+370) has surprisingly led his team to one of the best records in the West despite extended absences from his top two stars. Jenkins (+600) has done the same with the Grizzlies, whose stellar December has them knocking on the door of the West’s No. 1 seed.

Those two join Mazzulla and Vaughn as the only coaches with 6/1 odds or shorter at most books, while four other head coaches — Rick Carlisle (Pacers, +800), Michael Malone (Nuggets, +800), Mike Brown (Kings, +900) and JB Bickerstaff (Cavaliers, +900) — are all dealing at shorter than 10/1 as we approach the halfway point of the season.

It’s anyone’s guess which of those will emerge as legitimate candidates down the stretch, likely dictated by which squads can lock down a top seed in their respective conferences. For now, Mazzulla remains ahead of the pack, ever so slightly, amid one of the most impressive coaching jobs in recent memory.