Boston Celtics guard Kyrie Irving ranked 63rd and 57th, respectively, among point guards the past two seasons in Defensive Real Plus-Minus. (AP Photo/Charles Krupa)

Following the franchise’s most successful season under Brad Stevens, the Boston Celtics made history — by overhauling the roster. Four players remain from last year’s 53-win team, which, according to the Elias Sports Bureau, makes Boston the first squad in league history to pile up the most wins in its conference and return the following season with less than five pieces from the roster that accomplished it.

Danny Ainge’s alterations were made to challenge the Cleveland Cavaliers, the team that has governed the Eastern Conference over the past three seasons and eliminated Boston from the playoffs in two of them. Instead, it may have weakened the Celtics too much defensively.

Four of the team’s six best defenders, as defined by ESPN’s Real Plus-Minus, Jae Crowder, Amir Johnson, Kelly Olynyk and Tyler Zeller, are gone. Each played significant minutes for Boston a season ago and their departures will be sorely felt. Crowder, who now plays for Cleveland, and Johnson finished in the top 65 in DRPM and accounted for the team’s best on-court net ratings. All save for Olynyk ranked between the 75th and 100th percentiles in points allowed per possession, according to data provided by Synergy Sports. When each of the four players sat, opponents scored at rates higher than Boston’s season average. With Crowder and Johnson on the floor together, Boston allowed 102.7 points per 100 possessions, a figure that, if maintained over the entire season, would vault the Celtics from sixth in the Eastern Conference in defensive rating to first.

Player Points allowed when on the court Points allowed when off the  court
Jae Crowder 104.5 106.9
Amir Johnson 103.4 106.9
Kelly Olynyk 104.2 106.3
Tyler Zeller 104.4 105.7

Boston’s defense also clamped down on opponents from beyond the arc last season, allowing 0.971 points per possession on three-point attempts, the second-best mark in the league. Opponents shot a below-average 34.2 percent on corner three-pointers, the third-most valuable shots in the game behind free throws and shots from the restricted area. Much of that was due to Crowder and Bradley’s defense. Crowder led all players last season (minimum 200 defensive possessions) in points allowed per three-point attempt, beating out LeBron James, defensive player of the year Draymond Green and first-team all-defensive selection Patrick Beverley for the top spot.

Isaiah Thomas, one of the worst defenders in the league, caused some of Boston’s issues defensively. His replacement, Kyrie Irving, is not much better, ranking 57th (compared with 63rd for Thomas) among point guards the past two seasons in Defensive Real Plus-Minus.

That brings us to the three-point line, which could go a long way in deciding Boston’s fate this season. There’s never been more of a premium placed on efficiency and production from beyond the arc, and, in turn, more of a premium placed on defending the shot. Boston was an above-average team last season — and has been since Stevens arrived — at chasing teams away from the three-point line, allowing 26.9 three-pointers per game, sixth best in the Eastern Conference. Thomas may have been a sieve on the defensive side, but he was actually better than Irving when it came to spot-up defense, mostly from beyond the arc; Thomas ranked in the 67th percentile last season, according to data provided by Synergy Sports, while Irving ranked in the 17th.

Last year was arguably Irving’s best defensive season and he still ranked 111th in points allowed per possession on three-point attempts (1.24). So Irving is about as much of a defensive upgrade as someone who ranks in the bottom-fifth of the league by most defensive metrics can be.

What happens when Thomas, Bradley and Crowder are replaced with Irving, Gordon Hayward and Jaylen Brown? Hayward benefited from an elite defensive ecosystem constructed by Utah Jazz Coach Quin Snyder, and Brown turns just 21 next week. It’s tough to imagine a scenario in which Boston improves defensively this season, especially when it comes to mitigating the three-point production of opponents.

In total, there are too many unknowns with this group; the four remaining players from last year’s team — Brown, Al Horford, Terry Rozier, Marcus Smart — played just 14 minutes together in the postseason. FiveThirtyEight projects a 47-35 record and an 86 percent likelihood of reaching the postseason, which puts Boston in line for the second or third seed in the East come playoff time. That’s a regression of eight wins and one or two spots in seeding, which seems about right considering what Boston chose to part ways with this offseason in search of a greater offensive ceiling. Cleveland is the barometer that the Eastern Conference measures itself by, and Boston has little chance of dethroning the Cavaliers with this defense.

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