Boston Celtics are up 1-0 in the Eastern Conference finals over LeBron James and the Cleveland Cavaliers. (Charles Krupa/Associated Press)

Boston has no business being in the conference finals, much less the NBA Finals. Its season started with a horrific injury to prized offseason acquisition Gordon Hayward, and then Kyrie Irving was lost for part of the regular season and all of the playoffs. Now the Celtics are up, 1-0, in the Eastern Conference finals over LeBron James and the Cleveland Cavaliers, and there is a reason they are thriving.

Without Hayward and Irving, the Celtics are a classic underrated team: They’re starless (well, Al Horford is an all-star, but he often flies below the radar), but their defense is elite and the sum of their parts will beat most teams. It even has a name, the “Ewing theory,” which postulates that teams are often better without their stars due to synergy and an increase in team-oriented play.

In 1999, the Knicks lost Patrick Ewing after a fairly lackluster season. Then they stormed through the playoffs, beat teams with superior talent, and made the NBA Finals, eventually losing to the San Antonio Spurs. The similarities aren’t perfect — Ewing was 36 at that point, and it was a lockout season — but it shows Cinderella runs can happen in the NBA playoffs.

And all of Boston’s injuries haven’t dulled one of their biggest advantages: Brad Stevens. The young coach has been praised heavily, but it’s hard to deny his impact. The Celtics had one of the best defenses in the league, and that carried over into the playoffs. They stopped the Philadelphia 76ers, one of the hottest teams in the league, and presumptive rookie of the year Ben Simmons was rendered mortal. He shot poorly and had nearly as many turnovers as assists, a terrible sign for a point guard. They knew how to guard him: Drop back, kill a driving lane and shut off his passing options. You can see an example of that below.

Boston has extended its smothering defense to LeBron James. Given their flexibility and versatility, the Celtics can switch aggressively and their defenders can quickly scuttle back to shooters. James was locked out of the paint, and while he’s obviously a better shooter than Simmons, taking a number of contested jump shots is a win for the defense. That is not how Cleveland is going to win.

When James actually got to the rim, help was usually there waiting for him, as you can see below.

There are some important reasons Boston was able to slow down the best player of his generation. It took advantage of Cleveland’s personnel and knew it could help off Tristan Thompson, who doesn’t have a usable jump shot, and stray far off Jeff Green, whose own jump shot is unreliable. This shrinks the floor, and it allows the Celtics to guard James closely and cut off his runs to the rim.

Second, Boston’s switches — a defensive tactic that makes it harder for shooters to get open — stalled Cleveland’s offense. Marcus Morris deserves a lot of credit for defending him well, but it was truly a team effort. Horford and Marcus Smart, who can play center and point guard, respectively, were guarding James at times. This is similar to the defensive success of Golden State’s so-called death lineup: The Celtics have a number of guys the same size or with great versatility, such as Smart’s strength against bigger players or Horford’s ability to defend the perimeter. James was unable to create mismatches because most of their entire team defended him competently, which was made possible through help defense.

For example, James averaged 42.7 points per 100 possessions in the first two rounds but was held to just 20.5 points per 100 when Morris was the primary defender. Against Semi Ojeleye that dropped even further to 13.3 points per 100. James had no points in seven possessions when Terry Rozier was the defender.

2018 NBA playoffs    Per 100 possessions
LeBron James Possessions Points Turnovers
Rounds 1 and 2 883 42.7 3.9
vs. Marcus Morris 39 20.5 10.3
vs. Semi Ojeleye 15 13.3 13.3
vs. Terry Rozier 7 0.0 14.3
vs. Jaylen Brown 4 0.0 0.0
vs. Al Horford 2 100.0 0.0
vs. Jayson Tatum 2 100.0 0.0
vs. Aron Baynes 1 0.0 0.0

And just like the Sixers’ Simmons, James had a turnover problem. Boston had impeccable timing on some of his passes, and the other turnovers were a result of James scrambling to create something. This led to one of his worst playoff games ever. In fact, by game score, which measures the production of a single game by weighing box score stats, it was one of his 10 worst postseason games.

While the first game was encouraging for Boston, there are some reservations. The Celtics have won a number of close games, and Cleveland missed a suspicious number of three-pointers. The Cavaliers have time to adjust, and James has as many tools as anyone in the league to counter their defense. But we shouldn’t dismiss the Celtics. They’re well-coached, athletic, they can hit three-point shots and they play defense with pinpoint precision. As strange as it sounds for a higher seed, this is an underdog the other team should fear.

Read more NBA coverage:

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What I got wrong about the Boston Celtics

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