The Eastern Conference finals series between the Boston Celtics and Miami Heat is chock full of stars. There are first-time All-Stars like Jayson Tatum and Bam Adebayo. There are old standbys like Jimmy Butler and up-and-comers like Jaylen Brown. And there are pick and roll maestros like Kemba Walker and Goran Dragić.
But these teams also have stars who spend most of their time on the bench: coaches Erik Spoelstra and Brad Stevens. Spo is one of only 14 coaches in NBA history to win multiple titles and one of only eight to do so since the ABA-NBA merger. His 0.623 playoff winning percentage is 10th-best in league history and fourth-best among 23 coaches who have stalked the sideline for 100-plus postseason contests. Stevens has not yet matched Spoelstra’s postseason accomplishments, but his teams have averaged 51.9 wins per 82 games over the past five years,1 and despite being in just his seventh season, this is already his third trip to the conference finals.
There are a lot of similarities between the two coaches as well. There’s tenure, for one: Spoelstra is the league’s second-longest-tenured coach (12 seasons) and Stevens is sixth (seven seasons). There’s their relative youth: The 43-year-old Stevens is the sixth-youngest active coach while the 49-year-old Spoelstra is the 12th-youngest. (They were hired at age 36 and 37, respectively.)
And of course, there’s the modest origin stories: Spoelstra started out as the Heat’s video coordinator and — according to his father, former NBA executive Jon Spoelstra — only got to keep that job because then-incoming-coach Pat Riley was not contractually allowed to hire his own video guy. Stevens, meanwhile, started as a volunteer in the Butler University basketball department and was preparing to work at Applebee’s before being offered an assistant coaching position.
But perhaps the most meaningful similarity between the two is actually what drives the performance of their teams on the floor: stifling defense. Spo and Stevens have each only coached one below-average defensive team in their career.
Spoelstra’s 2014-15 Heat squad ranked 21st in defensive efficiency after losing LeBron James in free agency, remaking the roster at the trade deadline and seeing Chris Bosh’s season (and, ultimately, career) cut short due to blood clots in his lung. In eight of Spoelstra’s 11 other seasons, the Heat have sported a top-10 defense. The 2013-14 Celtics finished 18th in defensive rating, but Stevens’s side has ranked inside the top seven in four of six seasons since. The average Stevens defense has finished the regular season 2 points better per 100 possessions than the average defense in the NBA that year, while the average Spoelstra defense has been 2.1 points better than average.
A quick look at the Celtics’ performance in each of the Four Factors during the Stevens era reveals their defensive success has been based almost exclusively on shot defense and turnovers.
Boston has allowed its opponents a better-than-average shot conversion rate in only one of Stevens’s seven seasons, while it has ranked in the top 10 in opponent effective field-goal percentage in each of the last five years. The Celtics have finished inside the top 10 in defensive turnover rate three times — and they’ve never finished lower than 18th under Stevens. Their performance in those two categories has kept them near the top of the league in point prevention despite consistently sending their opponents to the free-throw line and often getting beat up on the boards.
The Spo-era Heat, on the other hand, have alternated between counting on their ability to force turnovers and their ability to snag defensive rebounds. While they have also sent opponents to the line with greater frequency than most coaches would like, they’ve not been quite as friendly in that area as the Celtics; and during the Big Three Era of James, Bosh and Dwyane Wade (2010-14), they were actually quite good at avoiding fouls. (That’s generally been a staple of LeBron teams.)
But those are just results. There’s also the matter of the tactics that yielded those results. To examine this, we can turn to the new metrics we created in advance of the playoffs: Aggression+ and Variance+. Aggression+ is an index statistic that measures how much more or less aggressive than the league-average defense a particular team’s defense was in that season, both against every type of action2 and overall, while Variance+ measures how much more or less often than average that team changed its defensive strategy from night to night. And it’s there where we can see a bit of divergence between the two coaches.
Spoelstra’s 2013-14 Heat team was not only the most aggressive defense in the league that season, it was the most aggressive of the entire Second Spectrum era, which covers the 2013-14 through 2019-20 seasons. The most aggressive Stevens-coached Celtics team (2019-20), meanwhile, ranked 39th in Aggression+ out of the 210 team-defense seasons we examined. Spoelstra has also proved significantly more willing than Stevens to change up strategies from night to night. Three of the past seven Heat teams ranked inside the top 15 (out of 210) in Variance+, while only one of Stevens’s seven Celtics teams even ranked inside the top 100. (The 2015-16 Celtics checked in at 99th.)
Stevens’s teams, whether more passive (2014-15 through 2017-18) or aggressive, have tended to use the same strategies throughout the year. Spoelstra-coached teams have been aggressive and variable (2013-14 and 2014-15), passive and static (2015-16 and 2016-17), and everything in between. The fact that Spoelstra has coached so many different types of defenses while Stevens has largely used the same strategies with a greater degree of consistency hints at another difference between the two coaches: They have been tasked with leading wildly different rosters.
The Stevens-era Celtics have been composed almost exclusively of young players. The average minutes-weighted age of his Celtics teams is just 25.3 years old, per Basketball-Reference.com, and the oldest team he’s coached was the 2016-17 squad that had a minutes-weighted age of 25.9 years old.
To put that in perspective, consider that 10 of Spoelstra’s 12 Heat teams have been older than the oldest Stevens-coached Celtics team. The last two teams of the Big Three era had minutes-weighted ages nearly as old as the oldest player on this year’s Celtics, 31-year-old Brad Wanamaker.3 The two most youthful rosters Spoelstra has led were his first team (the 2008-09 team that had a minutes-weighted age of 25.6 years old), which featured such luminaries as a rookie Michael Beasley, and … this year’s Heat team, whose minutes-weighted age is 25.9 years old.
The team Spoelstra has put on the floor this postseason, though, is quite a bit older than the one he used during the regular season. The starting point guard minutes that had been going to Kendrick Nunn, who is in his age-24 season, are now mostly being soaked up by Dragić, in his age-33 season.4 Minutes that previously belonged to Meyers Leonard (age-27) are being distributed instead to Jae Crowder (age-29). Jimmy Butler (age-30) isn’t sitting out 20 percent of the games like he did during the regular season. Even 20-year-old Tyler Herro’s increase in playing time is being offset by significant minutes for Kelly Olynyk (age-28) and Andre Iguodala (age-36).
The Heat’s minutes-weighted age in the postseason has shot all the way up to 27.1 years old. The Celtics, on the other hand, have gone even younger in the playoffs.5 Wanamaker is playing a bit more, but so are Grant and Robert Williams (21 and 22 years old, respectively), plus Tatum (in his age-21 season), Brown (age-23) and Smart (age-25). And, of course, Hayward (age-29) missed 11 of the Celtics’ 14 playoff games.
And yet, true to form, each team is still playing solid defense. The league-average team scored 110.6 points per 100 possessions this season, and the offensive environment was considerably better in the bubble. The Celtics are sporting a tied-for-playoff-best 104.8 defensive rating, while the Heat check in at 108.6. There’s quite a bit of noise to those numbers because they’ve each only played specific teams, but each of their opponents scored at a higher rate during the regular season than they did against the Heat and Celtics in the playoffs.
The only opponents the Heat and Celtics haven’t been able to stop from scoring below their season averages so far during the postseason are the Celtics and Heat. Boston is at 113.9 points per 100 possessions for the series compared with 113.3 during the regular season. Miami is at 112.9 in the series compared with 112.5 during the year. As the series moves to Game 4 tonight, it feels safe to say that whichever team’s defense can lock in first has a good chance to tip the balance in their favor.
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