Boston's historic offense derailed by three-point shooting slump

On Dec. 9, Boston had an offensive rating of 119.9. They were shooting 40 percent from three-point range as a team, and were making 16.6 triples a game, which would have been the second-best total all-time. Their true shooting percentage was an all-time best 62.6 percent.

What's happened since then? Since the beginning of December, Boston has gone 8-8. They're shooting 32.2 percent from distance since then, and went from scoring 120.8 points per game to scoring 114.1 per game. Their free throw percentage dipped from 85.3 percent to 79.4 percent. The Celtics have gone from the best three-point-shooting team in the NBA to the 9th-best, but they're still putting up a ton of attempts - nearly 42 per game.

Some of this is simply regression to the mean. No matter how good Boston's ball movement and spacing might be, Marcus Smart and Derrick White are below-average three-point shooters for their careers. It's unrealistic to expect 36-year-old Al Horford to sustain his 44 percent shooting from deep. Though on the flip side, stars Jayson Tatum and Jaylen Brown were unusually poor from outside since November (32 percent and 33.3 percent respectively).

This wouldn't be as concerning if Boston still had last year's lockdown defense. But the absence of Robert Williams and the team's smaller lineups has led to a much more ordinary defense that's been outright bad at times, including a debacle Tuesday night against the Oklahoma City Thunder.

To add injury to insult, Coach Joe Mazzulla missed two games after scratching his corneas playing pickup basketball. 

The 26-12 Celtics now lead the surging Brooklyn Nets, who have won 12 in a row, by just half a game. Next is a tough road matchup against the Dallas Mavericks and Luka Doncic, who is scoring 50+ points every other game.

Boston isn't as bad as they looked on offense in December, but nor are they as good as they looked in the first quarter of the season. Perhaps the lesson is not to call a team the greatest offense in history until at least January.