It’s Nov. 6. The Boston Celtics, a team competing one of the East’s top spots, had just been blown out by the lottery-bound Denver Nuggets.

Isaiah Thomas tied a then season-high 30 points, but Denver’s Emmanuel Mudiay torched the Boston defense for 24 first-quarter points. It was 42-23 after one.

The Celtics didn’t have Jae Crowder, who was nursing a sprained left ankle. But they also didn’t have Al Horford, their marquee free agent acquisition over the summer who was out with a concussion.

Thomas couldn’t believe how soft his team’s defense looked in falling to .500 just six games into the season. Avery Bradley followed suit.

"These signs aren't good. We're not giving our all on the defensive end," Thomas said. "We can have five starters out (but) you can definitely, it's a mindset thing on that end, and we've got to do a better job on that."

Avery Bradley said the defense really misses Al Horford, talking on court and ensuring everyone's in sync and on same page. — Adam Himmelsbach (@AdamHimmelsbach) November 7, 2016

Fast forward two weeks and Boston’s defense has transformed into the imposing unit many believed it would when the team acquired Horford over the summer. The Celtics are 4-1 this season with their starting center on the floor versus 4-5 with him on the sidelines.

Boston is much improved on both ends with Horford on the floor, as ESPN’s Chris Forsberg pointed out. The Celtics are plus-47 when Horford plays versus minus-50 when he doesn’t. They score 111.9 points and give up just 97.1 per 100 possessions with their newly acquired big man on the floor, a +14.9 rating. Without him, Boston is outscored by 3.1 points per 100 possessions.

Why? Because Horford has filled in so many gaps for Boston on both ends of the floor.

An easy example of his dual-citizenship in the offense/defense club is his game-winning effort against the Detroit Pistons in his first game back.

Al Horford's return reminds us what the Celtics have missed, and what they can be: https://t.co/M6hDmwcljD pic.twitter.com/rKsuANlBB3 — Dan Devine (@YourManDevine) November 20, 2016

But take a look at how Boston is defending with and without Horford patrolling the middle.

Without Horford, Boston allowed Denver to score 18 of their first 23 points from within five feet. Mudiay relentlessly attacked the rim, finding little-to-no resistance in the paint. Had Horford been in the game on this play, Mudiay’s shot attempt would have likely been denied.

When the Celtics adjusted and packed the paint, the Nuggets began showering them from the perimeter.

But take a look at Boston’s Monday night game with Horford back in the rotation. The Celtics held Minnesota — one of the league’s better three-point shooting teams — to just four makes on 24 attempts from downtown on Monday. Overall, Boston held the T’Wolves to an offensive rating more than 10 points below their average.

Here, Horford showed one of his more valuable defensive traits: the ability to switch onto smaller, quicker opponents. Horford picked up Andrew Wiggins at the top of the key and forced a near-impossible lay-up attempt in a 13-0 run as part of a late, successful Celtics comeback.

Horford has been touted as one of the league’s supreme perimeter big men scorers for much of his nine-year NBA career. But it’s his effect on the defensive end that has Boston poised to challenge Toronto — or Atlanta, Chicago and Charlotte early in the season — for the East’s second spot behind Cleveland.

And if he keeps making plays like this, Boston’s shaky start will be water under the bridge come playoff time.