The game opened with LeBron James hitting JaVale McGee on an alley-oop from half court, followed by an ugly Jayson Tatum drive. Then came a poor Boston defensive rotation, leading to Gordon Hayward's foul of Danny Green on a made 3-pointer. After a Kemba Walker drive was swatted away, Anthony Davis caught a lob at the front of the rim and casually slammed the ball home.
The Celtics were down 8-0 in an instant, with some of their biggest weaknesses on display against their forever rivals.
But then the Celtics started to look like their old selves. Rather than the team that lost six of its first 10 games to open the calendar year, Boston recaptured its early-season magic and blitzed Los Angeles on the way to a 139-107 win.
Lately, these kinds of impressive performances have been rare.
"[It was] a reminder of how good we can really be and how good we really are and how we were playing earlier in the season," Walker said after the win. "We had great energy, pressured the basketball, just having fun, playing with enthusiasm and passion.
"I think we got away from that a little bit."
Heading into Monday, the Celtics had beaten only the lowly Chicago Bulls (twice), Atlanta Hawks and New Orleans Pelicans this month. And, though they still rate as a top-six offense and defense for the season overall, the Celtics have sunk to league average in both categories since Jan. 1.
Here's what has been ailing Boston and whether the team can fix it heading into the 2020 NBA trade deadline on Feb. 6:
1. Lack of size
Monday night's opening minute was a reminder of the one glaring weakness Boston has had since this roster was put together: a lack of interior size. McGee's and Davis' size and length overwhelmed Boston on both ends.
Before that, the Phoenix Suns' Deandre Ayton went for 26 points and 15 rebounds on Saturday. The Philadelphia 76ers' Al Horford had 17 points, 8 rebounds (including five offensive) and 6 assists against his old team a week and a half ago. Joel Embiid has dominated Boston inside twice this season. All three had success in a similar manner: bullying the Celtics inside, simply overpowering the much smaller and lighter players trying to stop them.
"They pounded us on the interior," Celtics coach Brad Stevens said after facing Horford and the Sixers on Jan. 9.
Not only are the Celtics starting one of Jaylen Brown, Tatum or Hayward at power forward but they also don't have a defensive center capable of credibly holding up against the league's best bigs.
That's not to say Boston hasn't had solid play at the 5 this year. After dealing with a knee injury last season, Daniel Theis has been a revelation as a quality starting center. The Celtics have a plus-6.4 net rating with Theis on the floor. But for all that he does well, he doesn't have the heft to slow down players such as Embiid or Davis.
2. Defensive slide
To make up for that expected size deficit, Stevens built a defense that leans into Boston's strengths: speed, smarts and athleticism. That was largely working before this latest skid, with Boston entering 2020 with the league's third-best defensive rating. However, the Celtics have the No. 12 defense this month -- and they often look worse than that.
That can partially be traced to poor shooting luck, as Celtics opponents entered Monday with an effective field goal percentage 4.13 points better than expected in January given their shot quality, per Second Spectrum data. That was the third-largest margin in the NBA.
But there's more going on here.
"We're just not really running our system on the defensive end," Marcus Smart said after the Celtics' loss to Phoenix on Saturday. "We gave up a lot of easy, easy layups at the rim. Guys are supposed to be pulled in, and it's like we don't know what we're doing out there, and that's a problem. We're not really holding guys accountable on the defensive end."
One issue is that the way Stevens has the Celtics playing leaves little margin for error. To execute their scheme -- which often relies on the players on the court sussing out a mismatch and quickly rotating to eliminate it -- the Celtics need to be playing "on a string," as their coaches are fond of saying. That means all five guys have to know what to do and when to do it or the system will collapse -- as it had over the past few weeks before Boston beat L.A.
"Everything. Like, just everything," Stevens said Saturday when asked what needs to improve. "There's 50 things to do on a possession. Right now, we're doing about 46 on our best ones. And we need to do 50.
"This will be a good stretch when we look back on it, because it will force improvement. It'll force urgency on every detail, it'll force you to do your job for 48 minutes. These are never fun to go through. It sucks. But this is usually what you look back on and say it was a springboard for you."
And for the Celtics to maintain Monday night's level of play, they'll have to overcome another obstacle or two that have dragged them down lately.
3. Depth and tired legs
Boston's top six players -- Walker, Brown, Tatum, Hayward, Theis and Smart -- can hang with the NBA's best squads. Enes Kanter is a fine backup center most nights, too, as he showed with 18 points and 11 rebounds against the Lakers.
Beyond that? Well, there are only questions. Of Boston's three rookies, only Grant Williams has been a consistent part of Stevens' rotation. Semi Ojeleye gave Boston solid minutes Monday -- he was one of just eight players who played in the competitive portions of the game, as Stevens went with a playoff-like rotation -- but he doesn't provide much offense. Brad Wanamaker's shot has fallen off a cliff. Robert Williams is out indefinitely with a hip issue.
The lack of depth seen via the eye test is backed up by the data. The Celtics and the Philadelphia 76ers are the only two teams in the league to have five players (Walker, Brown, Tatum, Hayward and Smart) averaging more than 30 minutes per game while amassing a usage rate of greater than 15% in at least 25 games played. Couple that with Monday marking just the 11th time in 42 games this season that Walker, Brown, Tatum, Hayward and Smart have all played (due to a variety of injuries) and it's easy to see why Boston would have some tired legs.
"I feel like we're tougher to guard when myself, [Brown], Kemba, Smart, when everybody's out there," Tatum said after scoring a game-high 27 points Monday night. "It makes it tougher for the other teams to guard, and it makes it easier for all of us with so many guys that can do so many different things out there."
Having their full perimeter rotation available for an extended period will certainly alleviate some of these issues -- especially on defense.
"I want to see if we can play well with a sustained period with more bodies available," Stevens said Monday. "Then we can gauge where we are, how good we are and all that stuff. But there's still a lot of unknown. This is one game."
Even at full strength, Boston might be a player short. So, can the Celtics get one quality rotation piece -- or more -- before the trade deadline or via the buyout market?
It's hard to see how. Under president of basketball operations Danny Ainge, Boston has generally been reticent to make trades at the deadline. Acquiring Isaiah Thomas in 2015 was the only one of consequence in recent memory. The relative lack of activity around the league and Boston's dearth of tradable salaries -- the top five wings are their only players making more than $5 million, and none is likely to be moved -- make it unlikely Boston will change course there.
That leaves the buyout market, which provides no guarantee of success. Most of the time, players who change teams between the deadline and March 1 fail to provide much of an impact. Still, that might be the Celtics' best avenue to find a contributor good enough to deepen their rotation for the postseason -- and give them their best chance of hosting these Lakers again in June.