It wasn’t a great start for them — OK, the Warriors aren’t going 80-2 — but Golden State is back at the top of the Western Conference, as expected.

The Rockets, only somewhat surprisingly, are hanging in with them, though perhaps more because Chris Paul hasn’t played. The Pistons are a mild surprise, though many expected them to at least make the playoffs, and the Magic are a bit bigger surprise with their fast start. Though perhaps the most interesting question in the first month of the NBA regular season is whether the Cleveland Cavaliers have even the slightest chance of getting back to the NBA Finals again.

The story of the young 2017-18 season has been the Boston Celtics, winning a dozen straight after Sunday. Kyrie Irving didn’t play in that game, and, of course, major free agent acquisition Gordon Hayward is out for the season with a fractured ankle. Al Horford did just return from a concussion, and if they lose any more guys the Warriors might just surrender the 2018 title to the Celtics before the finals.

But the real reason the Cavs and LeBron James are in real serious trouble in the Eastern Conference is the way the Celtics now play, the league’s best defensive team, one that makes Kevin Garnett ask for pointers.

As an aside, Brad Stevens basically has wrapped up the Coach of the Year award. You can write than one down in ink now.

The Celtics are on the way to posting the best defensive efficiency rating (per 100 possessions) since the 2005 champion San Antonio Spurs. The Celtics — and I’m not sure I’ve even heard of some of the guys they are playing now — are first in the NBA in fewest points allowed per game (94.0). And by a lot, more than four points per game fewer than the second-place team.

They’re second in opponent field goal percentage (42.8), and first in defense against 3-pointers. And after being one of the league’s poorest rebounding teams last season, ranking 27th, they’re now even fourth in rebounding.

No team has ever gotten so much better by losing the All-Star free agent prize they’ve eyed for years.

And this after giving up their two best defensive players, Avery Bradley in the trade to Detroit for Marcus Morris, who just returned from injury, and Jae Crowder in the big trade to Cleveland for Kyrie Irving. The Bradley deal was to create the salary space to sign Hayward.

Oh yes, Irving. That’s no small addition, who now, of course, is out with a facial fracture and will likely have to wear a mask when he returns.

Irving has been terrific offensively, as usual, one of the true NBA magicians with the ball. He’s averaging 20.3 points per game, though the surprise, maybe the most unexpected of all, is Irving’s defense. Which has been the best of his career, if only because he’s actually playing defense for the first time. Normally ranked among the poorest defenders among point guards, Irving now is top five.

He must have really, really hated LeBron.

Anyway, a lot of that Irving defensive improvement, obviously, has to do with Stevens’ coaching, his unusual combination of tactical excellence with motivation while behaving like a normal human being on the sidelines.

But this may really be the Isaiah Thomas factor, which may be more trouble for the Cavs and may just account, in large part, for why the Celtics are playing as they are.

Thomas was great last season, it’s generally agreed. He was third in the league in scoring at almost 29 per game, the top fourth quarter finisher, an All-Star and inspirational story of the little guy who then loses his sister tragically in the playoffs and plays through injury as well to the conference finals. And then the Celtics trade him!

It’s a business, we know. And that may have been one heck of a business decision made by GM Danny Ainge.

Yes, all that winning the Celtics did last season was fun. But it’s no fun playing with a guy like Thomas. Sure, he can score, but it takes a lot of shots for him to do so and a lot of dribbling around. And what you have to do is always cover for him on defense because hardly anyone was worse (he’s maybe 171 cm) and run around to get his misses. It’s what generally wore out teammates playing with Allen Iverson.

Apparently the Celtics realized.

It was pretty much agreed their summer moves were aimed at the 2018-19 season, and once Hayward was hurt that seemed certain. After all, rookie Jayson Tatum and second-year man Jaylen Brown were now key players And a bunch of Aron Baynes, Semi Ojeleye, Shane Larkin and Daniel Theis were the support. But suddenly they were long and athletic, could help and switch.

Not only were they able to, but willing. Because everyone did so. Marcus Smart remained, an NFL-like tackler in a basketball jersey, and Al Horford, reliable on both sides, if often unappreciated.

Meanwhile, that Cleveland remake looked reasonable. They get the Nets’ draft pick, which could be top five, Jae Crowder, a top Celtics defender and the high-scoring Thomas, who’ll probably return around Christmas.

Cleveland added Derrick Rose and Dwyane Wade, which sounds pretty good. Until you watch them defend. Or not very much. You mean Dwyane Wade is arguing about a call again! The Cavs give up 20 more points per game than the Celtics and are tied for last with the Bulls in defending shooters. When Thomas returns, yes, the Cavs likely can only get worse on defense, if possible. They’ll likely play Iman Shumpert less, and he’s their best defender, but a poor scorer.

Hey, maybe LeBron will try to sign with the Celtics. We know, Kyrie, it’s your worst nightmare.

Sam Smith covered the Chicago Bulls for 25 years with the Chicago Tribune. He is the author of the best-selling book “The Jordan Rules.”

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