The Boston Celtics were supposed to be better than this, weren't they?

It's a bit of a funny notion, considering they're currently the 3 seed in the Eastern Conference, but coming into the season, expectations were high -- perhaps unreasonably so.

The current logjam in the East means that, at 15-12, the Celtics may be in third right now, but they're just 2.5 games ahead of the 11th place Washington Wizards.

This is a Celtics team that grew significantly in 2015-16, emboldened by their wunderkind coach Brad Stevens and riding on the small but mighty back of point guard Isaiah Thomas, a first time All-Star. When they added a premium free agent big man in Al Horford, many predicted the the Celtics would instantly become the Cleveland Cavaliers' biggest competition in the Eastern Conference.

Yet, they haven't separated themselves from the pack -- not even a little, which is a tad surprising a third of the way into the season.

So what's the story? It turns out, three things.

They've Been Injured

It's common knowledge that this Celtics team has been injured, but I don't think we properly credit just how decimated they've been.

Horford has played in just 17 of 27 games, Thomas recently missed four of his own, Jae Crowder has missed eight, and Kelly Olynyk missed six to begin the year.

The problem here is continuity. For a team that's integrating such a major new piece in Horford, their most used line-up (Avery Bradley, Crowder, Horford, Amir Johnson, and Thomas) has logged just 139 minutes together over 12 games.

For comparison's sake, the most used five-man lineup in the NBA is the Minnesota Timberwolves' lineup of Gorgui Dieng, Zach LaVine, Andrew Wiggins, Karl-Anthony Towns, and Ricky Rubio. They've played 448 minutes together over 20 games.

Now, Boston has a deep team, so they're subbing in and out more frequently, but they only have one other five-man unit that's played more than 50 minutes together so far. They've been a patchwork squad, and that can't help.

They've Actually Improved

Boston is sitting 10th in true shooting percentage (54.7%), up from 21st a year ago, and 9th in effective field goal percentage (51.1%) after finishing 24th last season.

Their assist to turnover ratio was already excellent, but it has improved again to second in the NBA (1.99), trailing only the Golden State Warriors. They also rank second in assist percentage (64.2%) and assists per 100 possessions (18.7), as opposed to last year when they finished 5th, 3rd, and 10th in those categories, respectively.

They've slowed their pace, an area where they finished third in the NBA in 2015-16, by nearly three possessions per game to 98.32, putting them into the bottom half of the league.

Their offensive rating has risen by 2.1 points, placing them ninth at 106.0 per 100 possessions. However, their defensive rating has risen in tandem, and their net rating for 2016-17 (2.2) is actually slightly worse than last season.

They're playing more deliberately and efficiently and they're moving the hell out of the ball, yet they're still struggling overall. Curious.

Really, it all comes down to one thing.

They Can't Rebound

It's no accident that you hear Boston, with their war chest of assets, connected to every available big man on the trade market: DeMarcus Cousins, Andrew Bogut, Nerlens Noel, even Greg Monroe.

If he's close to seven feet and can chew gum, the Celtics are in.

Does anybody have Hasheem Thabeet's number? No? That's probably for the best.

Boston tied for 18th in rebound percentage (49.4%) in 2015-16 but has been far worse this year. Through 27 games, they're 27th in the NBA at 47.7%.

You can't blame the absence of Horford for this either. Despite the fact that he's averaging a career low 6.4 rebounds per game, that most-used five man unit we mentioned earlier? They're the Celtics' second-best rebounding lineup this season, and their 47.7% is identical the Celtics' atrocious overall mark.

That ranks them 36th out of 41 five-man units that have played at least 100 minutes together so far this season. It's not just one thing either. The team is equally bad on the offensive (24th) and defensive (26th) sides of the floor.

Horford's Presence

It seems as if Boston fans thought the addition of Al Horford on its own would push this team to the next level. He was billed as an all-around power forward/center combination who could stretch the floor and play lights-out defense.

Even though he's been a fantastic team defender as usual and is averaging career highs in both assists (5.4) and blocks (2.3), adding Horford didn't address any of the things that Boston was actually bad at last season.

They needed rebounding -- Horford, despite his elite big man status, hasn't been a particularly good rebounder since 2010-11. His one excellent year since then (10.2 per game in 2012-13) was coupled with a massive rise in minutes played and perhaps fooled some casual fans.

Horford is very good, but adding him was like spending all your money painting a car that doesn't have wheels -- it looks nice, but at some point, you're gonna need the damn wheels!

The Celtics are moving the ball more, but they were already good at that. They still have trouble scoring at the end of games, and their lack of rebounding hurts them on both ends by making the game that much harder -- they rank 23rd in second-chance points given up (14.0) and 26th in second-chance points scored (11.9). When your point differential is only 2.2, that makes a huge difference.

Moving Forward

Boston will benefit from actually having their best five playing together as the season progresses, but they still need help on the glass if they're going to approach the expectations set out for them before the year began.

Our algorithm gives them a 93.7% chance to make the playoffs but currently views them as just the 11th-best team in the NBA.