Interior defense should be at the center of the Celtics’ thoughts

Daniel Theis makes like the man he was playing in place of, Robert Williams, in blocking this shot in the second quarter of the Celtics' victory Friday.Barry Chin/Globe Staff

The Celtics are learning that nothing is going to come easy in these post-Robert Williams days, especially on defense, where they are reverting to 2021 form with the lack of consistent stops.

Now an optimist would say the Indiana Pacers hit those same exact difficult and improbable shots in their February win against the Celtics, Boston’s worst game in months, one that snapped a nine-game winning streak.

But then again, these shorthanded Pacers, who gratefully for the Celtics lost their best player to foul trouble early in the fourth quarter Friday, sliced through the opposing defense with relative ease.

The Celtics had no problem scoring in their 128-123 win at TD Garden but they could never get consistent stops, and the Pacers reached a point where they knew they couldn’t be stopped. Boston decided to double Tyrese Haliburton and he simply hit the big man in the middle, who passed to a cutting forward for an easy layup.

If Indiana didn’t score at the rim, the Pacers were hitting one of their 17 threes. They shot nearly 52 percent from the field and 47.2 from the 3-point line.

The consistent theme after Friday will be that the Celtics need to make more adjustments to compensate for the absence of the league’s second-leading shot blocker. This is a more urgent situation than the race for the No.1 seed because the Celtics can beat any team in the Eastern Conference if they play sound defense.

“Communication wise, that’s a few games in a row that we’ve been as sharp as we need to be,” Celtics coach Ime Udoka said. “That was a little bit more apparent [Friday], just like last game. You would like to play better basketball so you’re not in that [close-game] situation, but it’s going to be tight games, close games that we need experience to go through.”

The positive is the Celtics have been lousy (4-15) at games with outcomes 5 points or fewer before Friday. And they continued to counter Indiana’s heroics with clutch baskets, including a soaring dunk from Jayson Tatum and last-minute scores from Jaylen Brown and Derrick White.

But the biggest responsibility in the last weeks of the season is not necessarily winning every game but ensuring they are playing their best basketball when the postseason begins in 14 days. For Udoka, that meant trying a double-big lineup with Daniel Theis and Al Horford that had moderate success.

But the Celtics are going to have to improve on allowing dribble penetration because an unimpeded ball handler in the paint causes all types of trouble. Haliburton, who scored 30 points in 26 minutes on 10-for-11 shooting, was drawing double teams. He countered by driving to the basket for floaters or hitting the big man in the paint, who would pass to a cutting Pacer for an easy layup.

The defensive breakdowns were concerning because the opposing talent level will only increase as the season progresses. Indiana was missing several contributing players, but Haliburton and backup center Jalen Smith scored at will at times.

What was considered a breather after a difficult two-game stretch against the Toronto Raptors and Miami Heat turned in a suspenseful battle because of the lack of defense or difficult Indiana shot making.

The Celtics’ best option is to use this as another learning experience, realizing that their vulnerability has increased with Williams recovering from a torn meniscus. Udoka pointed to the communication issues, the “You got him,” “No, you got him” talking on defense that led to a couple of easy buckets.

It’s going to take Udoka running out different rotations but also the players getting over the shock of Williams’s absence to improve in that category. They have enough good defensive players — when they are engaged — to come close to that dominant defense team that won 24 of 28 games.

The Pacers perhaps caught the Celtics by surprise with their free-style, 3-point happy play, but the more they get accustomed to playing without Williams, the better off they’ll be. Theis is the key component here, is Theis, and his adjustment to boosted minutes and roles.

“It was just one of those games, not every game is going to be pretty,” guard Marcus Smart said. “Obviously there’s a lot of things we can clean up, a lot of things we did good, a lot of things we did bad. We cleaned it up [our past mistakes] over the second half of the season. That team we played didn’t have anything to lose so they had so much freedom. It’s going to happen again in the playoffs and we’ve got to figure out ways when it happens to combat it.”

The Celtics face the Washington Wizards on Sunday, another game against a lottery-bound team that will be a challenge. Yet, it’s another opportunity for the Celtics to try to return to their former selves. No win, especially during their crucial stretch run should be taken for granted. Philadelphia lost at Detroit. The top-seeded Heat had lost four consecutive games until last Saturday.

Yet, Friday gives Udoka a lot of examples of slippage he can show his team over the next few days, in hopes they tighten up their coverages expeditiously.