Playing their third game in four days and missing Al Horford, who got the day off after logging 37 minutes in Saturday's win against the Pistons, the Celtics failed to match the Pacers' energy or even come close to it.
That led to Boston's defense, which is establishing itself as the best in the league, often being a step behind, struggling to stay in front of the latter's up-tempo offense.
Indiana consistently generated quality shots that it took full advantage of, knocking down 52.2 percent of its field goals, including going 17/33 (51.5 percent) from beyond the arc. The Pacers also produced 52 points in the paint, 20 on fast breaks, 17 at the free-throw line, 14 off turnovers, and 14 on second-chance opportunities. Four of their five starters scored at least 20 points, including Oshae Brissett, who had a game-high 27 points.
Conversely, tired legs led to Boston shooting just 12/43 (27.9 percent) from long range in Sunday's defeat.
Fatigue was a significant factor but certainly doesn't tell the whole story from today's loss.
Pacers' Play with More Energy, Shoot Lights Out in the First Half
The Pacers put 66 points on the board in the first half. They shot 52.3% from the field, including a scorching 12/20 (60 percent) from three. A Tyrese Haliburton swish from the logo late in the second quarter exemplified Indiana's hot shooting.
Beyond that, Boston had a hard time keeping up with the Pacers' up-tempo offense, as the latter pushed the ball off makes and misses. Even when the Celtics got set defensively, they had difficulty containing Indiana off the dribble, and they were often scrambling. The result was three of Indiana's starters scoring in double figures, led by Haliburton, who had a game-high 16 points at the break.
Perhaps, being the slower team was, at least in part, a product of this being the second half of a back-to-back and Boston's third game in four days.
The Celtics' lack of energy further suggests that was a factor, as does them shooting just 7/24 (29.2%) from three, though they did have 24 points in the paint and shot 46.8 percent from the field to prevent the game from turning into a blowout. Boston entered the half trailing 66-55.
Celtics Tread Water in the Third Quarter
Despite 10 points in the third quarter from Jaylen Brown, the Celtics generating 16 in the paint, eight off turnovers, and six on fast-break opportunities, they went into the fourth trailing by 11, the same margin they were down at halftime.
Twice, they reduced the deficit as low as seven points, but then the pendulum would swing the other way. What made that even more frustrating is it was the one quarter the Pacers cooled off from beyond the arc, only making two threes in the third frame. But Indiana added seven points at the free-throw line to help overcome that.
One of those points at the charity stripe came after Brown got a technical for shoving Goga Bitadze. The latter bowled the former over to establish position on the low block, and the lack of an offensive foul call on Bitadze prompted Brown to take matters into his own hands.
Oshae Brissett, who had 13 of his 27 points in the second quarter, stayed hot in the third, tacking on 12 more and hitting four of his seven shots, including both of the Pacers' made threes in the period.
Pacers Halt a Fourth-Quarter Run by the Celtics, Then Pull Away
While the second quarter was Boston's worst on Sunday, the fourth was marginally better. The Celtics got outscored 39-25 in the former and 35-25 in the latter.
A trying sequence came after Jaylen Brown hit a three to cut the Pacers' lead to seven with 8:14 left to play. Following that, Buddy Hield hit a three over Jayson Tatum and Grant Williams from the top of the key. On Boston's next possession, Hield came from behind to steal an entry pass to Jayson Tatum at the free-throw line, leading to a 2-1 fast-break and a layup for Hield at the other end to put Indiana ahead by 12.
The Celtics never threatened to steal this game after that, and with 2:54 remaining, Ime Udoka emptied the bench.
After taking two out of three on the road, the Celtics return home to host the Atlanta Hawks on Tuesday night. The game tips off at 7:30 ET. Inside The Celtics will have content related to the game coming out before, during, and after. And follow @BobbyKrivitsky on Twitter for updates and analysis from pregame to post.