Forty-six points. That’s the number Brad Stevens will have seared into his mind after the Boston Celtics’ 120-111 loss to the San Antonio Spurs on Monday night.

Boston exited halftime at AT&T Center leading 52-46, but the Spurs doubled their point total in the third frame, lighting up Boston’s defense both inside and out to the tune of 46 points. San Antonio continued to shred Boston in the fourth quarter, scoring another 30 points en route to the win.

The Spurs’ third-quarter thrashing of the Celtics saw San Antonio shoot 18-for-24 from the field, including 5-for-5 from 3-point range. The third quarter was the Spurs’ highest-scoring quarter since 1993 and the most points the Celtics had surrendered in a single frame since 2000.

San Antonio got everything it wanted in the second half, as LaMarcus Aldridge punished Boston’s interior defenders, DeMar DeRozan got downhill and created for others and Derrick White tore the C’s apart in the pick and roll.

“Forty-six point third quarter you’re going to get beat,” Stevens said after the game, as seen on NBC Sports Boston. “They were tremendous. It started with them physically overpowering us, getting to the rim clearly like a mindset of, ‘we’re going to go to the rim and make you stop us from a physical perspective.’ And once we started really sucking in, then they started spraying it all over and making every shot. They did a great job of exposing us throughout that whole quarter.”

The 46-point third quarter from the Spurs comes on the heels of a 38-point second quarter the Celtics surrendered to the Memphis Grizzlies on Saturday night. Boston’s interior defense specifically has struggled of late. With Aron Baynes and Robert Williams out with injuries and Al Horford on a minutes restriction, Stevens has been forced to play Daniel Theis and Guerschon Yabusele more minutes and it has led to stronger bigs, like Aldridge, tearing up the smaller C’s front line.

“Just started really driving to the basket,” Kyrie Irving said of the Spurs’ gameplan in the third quarter, “we had no rim presence, we couldn’t really get a rebound and they had 46 in the third. You can’t win NBA basketball playing that way.

“We have to play twice as hard because we are smaller,” Irving continued. “They are just pounding it in going at our bigs. Making Marcus Morris guard fours and Al guard these big fives and you know they’re just shooting and launching at the basket and they’re just getting offensive boards so, you know, most of our lineups are featured with four guards so it makes our job on the boards a lot tougher and it’s been showing over the last few games.”

The Celtics’ defense, while statistically one of the better ones in the NBA, has been getting gashed at times of late. While Baynes’ presence on the interior is an important one, his absence shouldn’t cause this much of a drop-off. Boston has struggled to stop penetration at times and that has led to a number of easy baskets for the opponents.

With a number of home games coming up in January, the Celtics will need to shore up their defense if they plan to take advantage of a favorable schedule to open 2019.

Here are more notes from Celtics-Spurs:

— Jaylen Brown scored a season-high 30 points Monday in the loss.

— Irving suffered a scratched eye in the loss. The star guard will receive antibiotics, Stevens announced after the game.

— The 76 points the Celtics surrendered in the second half was the most they’ve allowed in a half this season.

— Irving had eight assists for the third straight game.

Thumbnail photo via Soobum Im/USA TODAY Sports Images