BOSTON — While you ponder the last time the Celtics were ambushed like this against a good team in a marquee game (OK, we’ll save you the research; the answer is way back on December 10, a 21-point loss to the Suns), here are five takeaways from Celtics-Bucks, East semifinals, Game 1:

1. Giannis is no KD

The task of guarding Giannis Antetokounmpo poses a different — if not greater — challenge for the Celtics’ top-ranked defense than did Nets All-NBA forward Kevin Durant. Boston collected lots of roses for its lockdown of Durant in the first round, presenting a thesis on how to D up such a dangerous opponent. Durant, however, is mainly a (lethal) jump shooter while Giannis is an 18-wheeler without brakes careening down the mountain. Therefore, the strategy needed a tweak for this assignment.

And what was the Game 1 verdict? Well, the good news for Boston is Giannis needed 25 shots for his 24 points; that meant the Celtics stole his efficiency and made him work through the double-teams. The bad news is Giannis is a wiser offensive player, his passing and court awareness are crisper, and that’s why his 12 assists were just as damaging, if not more, than his points.

Hey, when the two-time Kia MVP can feed assists to himself, which he kinda did by throwing it off the backboard and then dunking the “pass,” you feel helpless. “That’s just pure talent, pure instinct,” gushed Bucks coach Mike Budenholzer. And when Giannis can literally hurt your defenders, as he did to Celtics center Robert Williams with an errant kick to a sensitive spot, this is indeed a whole other challenge.

2. Earth to Tatum

Welcome back. OK, let’s be fair here: Jayson Tatum is entitled to a subpar performance. Especially since he has resided in another atmosphere for about four months now. And particularly since he’s coming off a blistering first-round series, one of the finest both-ends acts in Celtics history — which says a lot — with his scoring (29.5 ppg) and defense on Durant. Well, gravity gets everyone at some point, and for Tatum, that came in Game 1 when he finally cooled and appeared a bit flustered and definitely frustrated at times.

Tatum never hurt the Bucks; he scored six baskets with three turnovers. Quite telling was a sequence when, down 16 in the fourth quarter, he had the ball poked away and was whistled for a foul while he and Wesley Matthews wrestled for it on the floor. Tatum rose to his feet and shook an angry fist to the officials; on the next trip down the floor, he was still seething … and promptly airballed a shot.

Again, to be clear, Tatum has been one of the top five players in basketball since January; that coincides with the Celtics’ turnaround and surge. But he came up empty Sunday, joined in misery by co-star Jaylen Brown (seven turnovers, four baskets). Boston probably can’t beat the Bucks if this continues more than once.

3. But what about the Bucks’ D?

For a good portion of the year, the Bucks were passive on defense and at times lousy, especially for a defending champ that prioritized defense. Other than Jrue Holiday and Giannis, everyone coasted. Evidently, though, the switch flipped after the All-Star break, and this had much to do with the return of Brook Lopez from a back injury that surfaced on Opening Night and sidelined him until April.

Lopez was once again solid on defense Sunday, especially when Giannis was forced to the bench with foul trouble. Lopez had three blocks and his length and rim protection made the Celtics think twice whenever they drove the lane. In the first round against Chicago, the Bucks held the Bulls under 90 points twice in five games, and now, the Celtics once in one game.

It’s a stark turnaround that doesn’t get much spotlight, but Milwaukee’s about-face is one of the big factors in the NBA this postseason. The Celtics discovered this firsthand when they managed just 33% shooting and spent much of the game playing catch-up. “In a way, it’s good to get this dud out of the way, offensively,” said Celtics coach Ime Udoka.

4. Just how they Jrue it up

The matchup between Jrue Holiday and Marcus Smart initially raised fears that the two supreme defenders might hold each other scoreless. Well, one game in, it’s Jrue 1, Smart 0. Adding injury to insult, the 2022 Defensive Player of the Year left the game in the second quarter with a shoulder stinger, one that looked much worse than it was.

Smart returned to start the second half, and his tough afternoon continued. He only made one second-half basket, and meanwhile, Holiday poured 25 points for the afternoon, a handful on pull-up jumpers, and turned the ball over only twice in 36 minutes. He was almost as damaging against the Celtics as Giannis, mainly because Holiday stayed out of foul trouble, impacted the game on both ends, and added nine rebounds and five assists.

5. Boston’s loss could prove costly

It’s never a good sign to drop the series opener at home and lose that advantage. A bigger issue for the Celtics is how the Bucks just bought themselves some time while they await the return of Khris Middleton. When Milwaukee can win without someone who carries as much weight as Middleton, it’s feels like there’s a bonus attached. There’s no firm timetable for Middleton’s arrival, although the original prognosis has him missing the at least the first two games of this series, which would make Game 2 almost a must-win for the Celtics if Middleton indeed is gearing up for a Game 3 return.

As they look ahead to Tuesday, the Celtics are comforted knowing Tatum and Brown most likely will not combine to shoot 10-for-31 again. Still, the Celtics must find a way to slow Giannis — easier said than done — and take their chances when the ball goes to Wesley Matthews or Grayson Allen or Pat Connaughton for open looks. Of those three players, only Allen is worth worrying about, based purely on how he’s played lately. Once Middleton is back in the Bucks’ lineup, Boston’s strategy of doubling on Giannis gets riskier because Middleton is a proven bucket-getter when left open.

Shaun Powell has covered the NBA for more than 25 years. You can e-mail him here, find his archive here and follow him on Twitter.

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