Why has the Celtics’ defense been so poor in second and third quarters?

Brad Stevens’s Celtics blew a 28-point second-quarter lead Saturday night.MICHAEL DWYER/ASSOCIATED PRESS/Associated Press

Even as the Celtics soared to a 28-point second-quarter lead against the Clippers on Saturday before coasting into halftime with a 21-point cushion, coach Brad Stevens was uneasy. He had seen his team begin to fall back into bad habits, and he sensed the danger in that.

“We knew it was going to be closer than a 21-point game with the way we played defense in the second quarter,” he said.

Stevens was prescient, but that was not enough to stop what was coming. With point guard Kyrie Irving sidelined with a sprained right knee, the Clippers stormed back and took a 123-112 win. Although the catastrophic manner in which Saturday’s game unfolded was certainly unusual, the general flaws that were revealed were not, and Stevens pointed to them afterward.

“Our defense in the second and third [quarters] has been bad all year when you compare it to the first and fourth,” he said. “For whatever reason we are ready out of the gate and we’re decent at the end — not tonight — but we’re not very good in the middle. I need to go back and look at what I can do to help us manage those minutes better.”

The Celtics’ offense has been consistent regardless of the segment of the game. Its fourth-quarter offensive rating of 113.1 is its best of the four, and its second-quarter offensive rating of 109.7 is its worst, a gap of just 3.4 points per 100 possessions.

But the defense has been much more mercurial. Boston has an elite 99.2 defensive rating in the first quarter this season, but it plummets to 109.3 and 109.5 in the second and third quarters, respectively, before leveling out a bit at 104.5 in the fourth. Stevens is understandably concerned by these variations, and he vowed to address them, perhaps even with personnel changes.

“I think I need to look at myself first and figure out what I can do to help that not happen,” he said. “If that means we need to play different rotations, call different things, start differently in quarters than we are, whatever the case may be, there’s an answer out there and we just have to find it.”

The somewhat confusing part of the second- and third-quarter lapses is that the personnel groupings are quite different. The second quarter is usually keyed by the backups, while the third quarter is mostly guided by the starters. But there are some trends that can be found while looking at pairings in both parts.

The most glaring issues have come in the frontcourt. Starters Marcus Morris and Al Horford have sparkling defensive ratings of 96.7 and 97.2 in first quarters this season, but both figures nosedive dramatically after that.

Morris has a 111.3 defensive rating in the second quarter and a 111.6 in the third, while Horford has a 110.8 rating in the second and a 112.1 in the third. Horford and Morris have the two worst defensive ratings in both quarters among Boston’s regulars.

Horford, who turns 33 in June, has battled knee pain that sidelined him earlier this season and led to a minutes restriction for several games after his return. He has long been viewed as one of the team’s essential defensive cogs, but this season his team-worst 105.7 overall defensive rating has provided some cause for concern, especially since it falls so sharply after strong first quarters.

Gordon Hayward’s defensive struggles, meanwhile, have been consistent throughout the game, and more obvious to the naked eye than those of Horford and Morris. He has had trouble keeping opposing ball handlers in front of him at times as he looks to regain the athleticism he flashed prior to last season’s devastating ankle injury. He has made strides, but there are times when he remains a liability. He is tied with Horford for the worst defensive rating on the team.

Stevens is constantly looking for groups that thrive together. His decision to put Marcus Smart and Morris into the starting lineup on Nov. 26 has mostly been lauded, and it was certainly a boon to the offense. But it is clear there is still work to do when finding the proper combinations later in games.

When the Celtics’ defense has been gashed this season, it is usually related to some combination of Horford, Hayward, and Morris being on the court together. Of the team’s 16 worst defensive ratings among two-man groups with at least 250 minutes played together, 15 have Hayward, Horford or Morris in them.

Hayward and Morris have been Boston’s worst defensive pairing among regulars, with a 111.3 defensive rating over 446 minutes together. Hayward and Horford have been the second worst, with a 108.3 defensive rating over 528 minutes. Meanwhile, Horford and Morris have been the fourth-worst defensive group on the team, with a 107.6 rating over 762 minutes together.

When isolated to the troubling second quarters, the defensive numbers are even more grisly: Hayward/Horford (125.1), Hayward/Morris (116.7), and Horford/Morris (114.9) have been the team’s three worst pairings with at least 50 second-quarter minutes together. In third quarters, Horford and Morris have a team-worst 118.7 defensive rating, followed by Morris/Terry Rozier and Horford/Smart.

The common threads do not stop when it is expanded to three-man lineups, either. Hayward is in each of the four worst defensive groupings with at least 50 minutes played together in second quarters, and Horford is in each of the six worst third-quarter groupings with at least 100 minutes together.

So, what can Stevens do? Well, first of all, he can hope Aron Baynes’s foot contusion heals quickly. Baynes has an elite 99.6 defensive rating this season, and he has been a wall during otherwise troubling third quarters: He is in each of the top five two-man defensive groupings. The 10 best second-quarter two-man defensive groups all have at least one of Baynes, Jaylen Brown or Daniel Theis in them.

Stevens may look for creative ways to stagger Horford, Hayward, and Morris, and he also might consider inserting Semi Ojeleye into mid-game situations in which the defense has been consistently gashed. Ojeleye has been used sparingly this season, but his 95.4 defensive rating over 410 total minutes is tops on the team by 4 points per 100 possessions.

The Celtics’ loss Saturday after holding a 28-point lead capped perhaps the most disheartening two-game stretch of Stevens’s tenure. On Thursday, the Lakers stormed back from an 18-point deficit to defeat the Celtics at TD Garden.

“We had big leads in both games,” Hayward said. “[We need to] try to find what we were doing in those stretches and build on that, and look at the parts where we’re giving up a lot of points and not getting stops and try to improve that part.”