Why the Celtics should consider shaking up their starting lineup

On the surface, things couldn’t be going much better for the Celtics a quarter of the way through the regular season. The team has overperformed early with an 18-4 record thanks to solid and timely contributions up and down the roster, which have helped the C’s overcome offensive lulls that have cropped up repeatedly over the first few weeks. Defensively, the Celtics had been an early juggernaut, but they’ve slowly fallen back to earth in that department (104 defensive rating in past six games).

Boston’s season defensive rating has risen to 98.3 points allowed per 100 possessions, which is still the best in the league, but it’s about four points higher than it was a couple weeks ago. Some regression shouldn’t be considered surprising -- the Celtics were defending at a level better than the NBA’s top defensive team last year for the first few weeks of the year. However, a closer look at the team's lineup numbers showcases a troubling sign: the C's starting five may be their worst defensive unit.

Out of the Celtics’ 17 most commonly used lineups (more than 15 minutes of court time), Kyrie Irving, Jaylen Brown, Jayson Tatum, Marcus Morris and Al Horford rank dead last in defensive rating. The starting lineup has played the most out of any of Boston’s top lineups this year (85 minutes) and is allowing 117.8 points per 100 possessions, nearly 20 points worse than the team’s season average of 98.3. That 117.8 mark would put them eight points below the NBA's current worst defense (Sacramento Kings at 109 points allowed per 100 possessions).

The team’s offensive rating with that unit has been subpar as well (102.8 points scored per 100 possessions), giving the group a -15 overall net rating, the only negative net rating among the team’s top 17 most-used lineups. The sample size is obviously a bit limited 20 games into the season, but those numbers are alarming nonetheless.