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Al Horford is the most significant free-agent signing in Boston Celtics history. It's a lofty yet accurate statement that's nudged along by the NBA's all-time most successful franchise either drafting or trading for a vast majority of its Hall of Fame talent.

But just over half a season into his four-year, $113 million contract, it feels unlikely that the four-time All-Star will ever qualify for a fifth. Age and the understandable adjustment period that come with joining a new team help justify an undeniable drop in some telling numbers.

Horford has never grabbed fewer rebounds per 36 minutes, and his true shooting percentage has been lower in only one other season. Horford's win shares per 48 minutes and free-throw rate are below his career averages, per Basketball-Reference.com. He's already missed 10 games—nine with a concussion and one to witness the birth of his daughter.

Does his statistical descent relative to the rest of a stellar career make him a disappointment? Or is Horford's overall impact in Boston still decidedly positive and enough to validate a maximum contract?

As with most NBA-related inquiries, a quick look at some numbers won't do the subject justice; it's a divisive question that can't be answered with one word. Horford's influence is subtle. His individual contributions on a box score can't capture how he makes his team better on both ends. He's smart, adaptive and excels in spots most players at his position can only marvel at.

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According to ESPN.com's real plus-minus, at 30 years old, Horford still ranks eighth among centers and 32nd overall in net point differential. Despite the fact that he is tallying an unimpressive (though expected) 15.5 points per game, Boston's offense averages 113.2 points per 100 possessions with him on the floor and just 103.7 points per 100 possessions when he sits.

That's the difference between the Golden State Warriors' slaughterhouse and whatever it is the Chicago Bulls think they're doing.

"He can go down to the block if you try to switch a pick-and-roll. He's comfortable in the mid-range. He's comfortable at three," Portland Trail Blazers center Meyers Leonard told Bleacher Report. "Good passer, kind of making plays in that little window, as we call it, on the short roll in the free-throw line area. So he's very talented, a guy that's been around the league and knows what it takes to win."

On-off numbers are noisy, especially when they capture only 35 games. But Boston's offensive jump when Horford plays makes sense for several reasons. Let's start with his passing.

Horford's assist rate (24.6 percent) is both a career best and 10 points higher than his average after nine years in Atlanta. Long recognized as the abnormal big man who can read a defense, be patient in a scoring situation and almost always make the right decision, Horford's unselfish tendencies have helped catapult Boston's egalitarian offense to sights unseen in over 25 years.

If his current numbers hold up, he'll become the first center in NBA history to post an assist rate higher than 24 percent beside a turnover rate that's lower than 11 percent. For this season, he leads all centers in potential assists, assists and passes made. He's the ideal pickup partner.

Horford's strong play is understandably lost amid the Isaiah Thomas hoopla, but that's partly because he never ventures outside the offense to rack up all his assists. He'll often receive an entry pass on the block, wait for the defense to react and then go to work—be it with a soft baby hook over his left shoulder or a quick dish to an open teammate.

Horford produces 1.24 points per possession when opponents double him in the post, per Synergy Sports. That places him in the 90th percentile and has materialized as one of Boston's easiest avenues to put the ball in the basket.

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NBA.com

The scary thing is there's still noticeable room for improvement, and Horford has yet to fully grasp his role in Boston's offense.

Here's what Celtics head coach Brad Stevens said about where he thinks his highest-paid player can still get better: "I think the biggest thing with Al is continuing to figure out his spots. Like the other day, I thought he had a great move in the post when we needed a bucket in a four-point game [against Portland], and I was hoping that we would go right back to that look. Just figuring out, again, how we can maximize him on the court on that end."

It's ironic given how dominant he can be down low, but the Celtics are never more perimeter-oriented than when Horford is in the game. The percentage of both their shots and points behind the three-point line highest when he's on the floor, and their percentage of points in the paint is the lowest.

That's because the Celtics love utilizing Horford's rare ability as a big to create space for others. He's shooting 34.8 percent from beyond the arc on a career-high 4.4 attempts per game. Last year, his three-point rate spiked to 24.4 after he rarely ever ventured that far from the rim in earlier years. This season, it's up to 33.5 (higher than Portland's C.J. McCollum and Cleveland's Kyrie Irving).

In pick-and-pop situations, opposing big men either forfeit an open three or scamper out to the arc and leave the paint vulnerable for Boston's guards to cut right through. It's an unanswerable dilemma for every defense the Celtics have faced this season, particularly when Horford gets paired beside another three-point threat like Kelly Olynyk.

"He's a really good player. It'd be nice if he wasn't, but he is. I mean, he's got everything. He's got every skill set you want," Houston Rockets head coach Mike D'Antoni said. "We went after him hard in free agency, and he shoots threes and can drive and whatever you need from him. He's really good."

For all the wonderful possibilities Horford creates when Boston has the ball, his work on the other end is much harder to analyze. The Celtics have surprisingly struggled to get stops all season. They rank 22nd in defensive rating and allow enough points per 100 possessions with Horford on the floor to drop down to 28th.

Horford is not a bad defender or a liability in any way. He contests shots all over the floor and is posting the highest block rate of his career. Boston values Horford's ability to cover multiple positions more than the Atlanta Hawks did, and he switches screens on the perimeter more frequently than before. He still has enough foot speed to keep up with speedy ball-handlers whenever they think there's an open runway to the rim.

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NBA.com

Against the Rockets on Wednesday night, Boston switched almost every late screen to put Horford on James Harden. His work during that stretch wasn't flawless (Harden told Bleacher Report "it doesn't matter who's guarding me" when asked about Horford's defense afterward), but it did the job and was key in lifting Boston to arguably its most important win of the season.

"We had planned to [switch] late, and Al did a great job on James," Stevens said.

Horford's pick-and-roll defense has been consistent, and he executes every game plan. But even when he's in the right place at the right time, Boston has fallen victim to some tough situations that should even out as the season goes on. Boston's opponents make 44.2 percent of their mid-range shots when Horford is in the game. That number is way above the league average and should sink a bit eventually.

In a recent game against the New York Knicks, Derrick Rose caught fire with a string of pull-up jumpers, but the Celtics (and Horford) welcomed those shots by repeatedly dropping Horford back into the paint whenever Rose came off a ball screen. They refused to deviate from the strategy. Some of Rose's looks were wide-open, and that's not desirable, but Horford did a good job taking away driving lanes and forcing the least efficient shot in the game.

A solid understanding of where he needs to be, coupled with some of the league's better one-on-one post defense, hasn't been enough. Boston's defensive weaknesses are well-known (rebounding, a poor steal rate, spotty rim protection and frequent miscommunications in the half court), and some can't help but fall on Horford's shoulders.

"I feel like I'm doing OK, but the biggest reflection for my impact has to lie on the team," Horford said. "And as a team right now, starting with me, we just need to be a little better."

While his block rate is way up, Horford's steal rate sits at a career low, and his rim-protection numbers are way down. Opposing field-goal percentages within six feet of the rim were 10 percent below their average when Horford was a nearby defender back in 2014. Now, they're even, up from 49.4 to 60.1 percent. That's troubling.

"We have to do a better job holding teams to one shot. That's the first thing," Horford said. "And I have to do a better job protecting the rim. I can recall a couple instances where I need to be there and I wasn't impacting the ball as much as I'd like to. I know I have to be better on the defensive end."

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Only six other players 6'10" or taller are snatching fewer than 11.5 percent of all available missed shots, and he hasn't protected the paint as well as expected, but Horford's work in space is terrific and necessary. He remains a good defender.

His wingspan is a constant nuisance and allows him to contest jumpers while maintaining enough distance to avoid a blow-by. Is he perfect? No. But Boston is clearly a better team with him on the roster, and his overall impact resembles a solution more than a problem.

If he stays healthy, Horford's game also figures to age well. He can shoot from almost anywhere, feed cutters, open spot-up threats in his sleep and effectively sew up pick-and-rolls in various forms of coverage for the next few years. The decision to bring him on at the price Boston did was a wise and obvious one.

This conclusion doesn't even consider his off-court influence: the calming presence he provides in the locker room and the altruistic, professional aura he carries on the road and in the weight room. Those factors aren't quantifiable, but they matter and go a long way.

As was known way back in July, the Celtics still have a few holes to fill if they want to topple true contenders like the Cleveland Cavaliers and Warriors. Horford is the perfect center to have on board when/if they do.

Boston Celtics Insider Notebook

Jaylen Brown Squanders An Opportunity

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With the explosive Rockets in town, one night after Marcus Smart was removed from the huddle after cameras caught him in a heated verbal altercation with several members of Boston's coaching staff, Stevens decided to insert Jaylen Brown into the starting lineup.

On the same day Boston's rookie withdrew his name from Slam Dunk Contest consideration, citing the desire to focus on helping Boston for its stretch run, Brown picked up four fouls in his first 12 minutes guarding Trevor Ariza (not Harden).

Brown was solid in his previous four games, averaging 7.8 points (on 47.6 percent shooting) and 3.8 rebounds. He's Boston's high-upside variable over the next few months.

Isaiah Can't Get Beverley's Respect

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Since Avery Bradley went down with a sore Achilles seven games ago, Thomas is averaging 34.9 points and 6.9 assists, shooting 48.9 percent from the floor, 39.7 percent from beyond the arc and 91.5 percent from the free-throw line.

On Wednesday, the second night of a back-to-back against one of the better teams in the league, Thomas dropped 38 points and nine assists to help dig Boston out from a three-game losing streak. After the game, Rockets point guard Patrick Beverley gave Thomas credit but refused to acknowledge his sustained success.

"Last time we played him, we won. I think he had like 14 points. Today, he played well. He had 38," Beverley said. "I mean, I don't know. We're not in the East. Isaiah Thomas, like I said, he had a helluva game tonight. We played him the first time, he had 16 points. We played him tonight, he had 38, so you gotta give him credit. He's an All-Star. A lot of the offense is focused on him, and he played well."

(For the record, Thomas scored 20 points in Boston's first game against Houston.)

All quotes in this article were obtained firsthand unless otherwise noted. All statistics are from Basketball-Reference.com or NBA.com unless otherwise noted.