Diagnosing the Celtics problems since the All-Star break

After entering the All-Star break with a 42-17 record and the #1 seed in the Eastern Conference, the Celtics have stumbled out of the break. Since resuming play, Boston has a 7-6 record and they've fallen to the #3 seed in the East. There are multiple factors that have led to this drop in play, so let's take a closer look at some of them.
One of the main contributing factors to the Celtics struggles is the play of Jayson Tatum. Don't get me wrong, Tatum is having the best season of his career, averaging career highs in points, rebounds, and assists, showing consistent improvements each year he's been in the league. However, when looking at his numbers from before and after the All-Star break, Tatum has seen some serious drops.

The first 55 games of the season, Tatum averaged 30.6 points per game while shooting 46.4% from the field and 35.7% from three. In the 12 games he's played since the season resumed, JT has managed just 26.9 points per game, shooting 42.3% from the field and 29.2% from three. While the Celtics had a 40-15 record in the games Tatum played the first half of the season, they are just 7-5 in the games he has played in the second half. 

With all of that being said, I still believe in Jayson and would argue he should still finish in the top 5 of the MVP race. Despite his recent play, I expect Tatum to kick it back into gear, hopefully in time to take the Celtics on a deep run in the playoffs. First team All-NBA Jayson Tatum is hopefully coming soon. Another thing that has plagued the Celtics recently is their poor record in close games. In the 13 games they've played since the break, Boston has had 5 of their games decided by 3 points or less, posting a 2-3 record in those games. Over the course of the entire season, the Celtics are only 5-7 in those type of games. Comparatively, Philadelphia is 9-5 and Milwaukee is 6-1, which can explain some of the shifting in the standings recently.

While there is still time to make a late push at a better seed, Boston's late game execution needs to improve. With two of the best wing players in the league on your team, there is no excuse for the Celtics failing to get over the line in tight games against lesser opponents. Boston can't afford to be giving away any more winnable games if they want to at least have home court against Philadelphia in the second round.
The final major factor is the health of the entire team, especially Rob Williams. The Celtics have had lots of guys miss a game here or there which has been detrimental at times, but no one has missed more games then Rob. When Rob plays and he's healthy, the Celtics are a completely different team defensively. 

With Rob being allowed to play a roaming role on the back line of the defense, the other Celtics are willing to take more risks on the defensive end, knowing that they have Rob back there as a safety valve. But since Rob has only been able to play in 28 games this season, the Celtics defense hasn't been nearly as good. Boston needs their defensive anchor back and healthy if they want to get back to the level they were at last year on the defensive end.
Boston has all of the pieces in place to get back to, and win the NBA Finals. Some could even argue that the Celtics are improved compared to last year in terms of their rotation when healthy. If they are able to clean up some of the simple things and play to the level they are more than capable of, this team is destined to raise another banner this season.