In addition to getting completely healthy and accustomed to playing with each other, the No. 1 priority of the Celtics is to improve defensively. They have been getting battered lately by opposing offenses and there’s been only flashes of the effort and performance of last season.
On Wednesday against the San Antonio Spurs, the problem was rim protection and the Celtics are going to have to find a way to clog up the paint to prevent drives into the paint or have a big man present to prevent easy scores at the rim.
Opposing teams are shooting 69.5 percent from the field against the Celtics on shots from zero to 3 feet, meaning most layups and dunks are unimpeded or scored without much resistance.
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The issue is the dramatic difference between Tristan Thompson and Daniel Theis. The Spurs, like many teams, don’t bother to defend Thompson outside the paint because he’s one of the few players in the NBA who poses absolutely no threat from the 3-point line.
San Antonio center Jakob Poeltl disregarded Thompson and simply stayed in the paint to rim protect. That gave the Spurs a distinct advantage because he misdirected many shots, including a clean block of a Jaylen Brown dunk attempt.
In that game, the Celtics scored 52 points in the paint on 54 shots. They made less than 50 percent of their shots in the key. The Spurs, on the other hand, made 27 of 33 attempts in the paint. They were essentially playing pop-a-shot against the Celtics defense.
“The No. 1 thing in my eyes is we’re not holding down the fort, we’re not guarding,” said Celtics coach Brad Stevens. “We’re not guarding the lane; we’re not protecting the rim. We’re not at the basket. I’m not talking about our bigs, I’m talking about everybody.”
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That means the Celtics need to get better at stopping dribble penetration. If Thompson, Theis or Robert Williams are guarding a big who is spacing away from the basket to open the key, the other defenders have to stop their men from diving into the line.
Against the Spurs, the Celtics couldn’t stop second-year forward Keldon Johnson, who used his crafty moves to score at the rim with ease. His streak of 8 straight points in the second period changed the course of the game.
The Celtics jumped into a zone defense in the second half to slow down the Spurs, who scored 61 points in the second half. But for a team that doesn’t regularly play zone, it was a gimmick of sorts, one that the Celtics were embarrassed to resort to because they couldn’t stop the Spurs with good, ol fashioned defense.
“It’s just our attention to detail,” All-defensive first team guard Marcus Smart said. “Our effort just isn’t there. We got to fix it. There’s no reason we shouldn’t be able to guard guys, man-to-man. We have to go to a zone [defense] because we can’t guard. It definitely can be fixed and it’s going to take us to fix it. It’s definitely frustrating, but the good thing about this league is can turn it around, we have a game every night.”
The infuriating thing for Stevens and Smart is the Celtics do show spurts of being that stifling defensive team that held the Spurs to 32 points through the game’s first 18 minutes Wednesday night. Then the Spurs responded with 29 points in the final six minutes of the second period.
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The defense is not deplorable; it’s just not up to Stevens’ standards. They are 14th in the NBA in defensive rating but 22nd in opponent’s field goal percentage (47 percent) and 21st in opponents’ 2-point percentage, which means teams are scoring too easily at the rim and off the dribble.
But rim protection has been an issue for years and the Celtics haven’t improved that aspect. Robert Williams, the team’s best shot blocker, is stuck as the third center. Thompson is more of a rebounder than shot blocker while Theis blocked less than one shot per game.
That places more pressure on the Celtics wings and guards to defend. Kemba Walker can’t continually get beat off the dribble, the same with Jayson Tatum and Jaylen Brown. It’s going to take a collective team effort because elite defenders are hard to acquire.
Of course, the road to improvement will be difficult with the Lakers, a top five offense, the first game of a difficult stretch that began Saturday night and includes games vs. the Warriors, Kings, Clippers, Suns and red-hot Jazz. If the Celtics are going to emerge as an elite team in the Eastern Conference, they are going to have to improve defensively quickly. Otherwise they are going to languish in the middle of the pack, having to beat opponents by simply outscoring them.
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That’s not the method Stevens wants to use to win.
Gary Washburn is a Globe columnist. He can be reached at gary.washburn@globe.com. Follow him @GwashburnGlobe.