Make no mistake: Game 2 was a harsh lesson. Now the Celtics need to learn from it.

Jaylen Brown showed his frustration with the Warriors' Draymond Green (23), who succeeded in taking the Celtics off their game.Jim Davis/Globe Staff

SAN FRANCISCO — Again, the Celtics were burned by their own ineptitude in a critical stretch, a team that has championship aspirations was psyched out by Draymond Green early, and then was so careless with the ball, offering the three-time champion Warriors countless opportunities to build on their lead.

It was almost expected the Celtics would play a porous third quarter after fighting to stay close in the first half, but they even outdid themselves with how bad they were after halftime, transforming into a bunch of iso-ballers who didn’t trust their teammates.

They were too focused on the officiating, which was admittedly bad and one-sided at times, but that caused them to lose their focus, forget they were playing a high-level team in the NBA Finals, and they fell apart like a bad game of Jenga.

The 107-88 loss will now change the narrative of this entire series. The Celtics were considered the hungrier and more athletic favorites after Game 1, and the Warriors used the physicality of Green and the shot making of Stephen Curry and Jordan Poole to regain their swagger.

Golden State's Draymond Green was a physical presence all night long -- here, he goes toe-to-toe with Jaylen Brown. Jim Davis/Globe Staff

Golden State was shaken after its Game 1 loss, and the Warriors promised to be more physical and assertive, which they were. But that didn’t mean the Celtics had to take it and turn into patsies. They appeared satisfied with splitting the opening two games and taking home-court advantage.

They allowed a stunning 25-2 run between the third and fourth quarters. Jaylen Brown never looked the same after his second foul. Jayson Tatum get his 3-ball going but countered that with sloppy passes or his customary stopping completely in the paint when he gets a no-call.

“That’s been an ongoing theme in the playoffs so far at times,” Celtics coach Ime Udoka said. “Had our opportunities. Came out, jumped out up nine early. Then turnovers started happening. Let them back in the game. Then the offense wasn’t as crisp with the ball movement and passing. Like I said, 11 of our turnovers were live ball turnovers, nine of them out of steals. So [we’ve] got to be better in that category.

“For us it was more, that’s a disappointing quarter of course, but the first half was just as disappointing, only being down two and not playing good basketball at all frankly.”

The Celtics paid dearly for their sloppiness Sunday, 19 turnovers resulted in 33 points. Curry had his moments, but the Celtics will take 29 points on 21 shots. It was the 17 from Poole. It was the slew of fouls that put the Warriors in the bonus for the final six-plus minutes of the third quarter.

It was an opportunity wasted. The Warriors tried bullying the Celtics early — and they succeeded — until they got their offensive swagger back with difficult shot making and easy points off turnovers. The Warriors don’t need help to win in the NBA Finals. They’ve done so plenty on their own merit, but Sunday the Celtics reverted back to their early-season ways and never gave themselves a chance.

The series is even at 1-1. The Celtics stole home-court advantage and no longer need to win in San Francisco to win the series, but they gift-wrapped the momentum right back to the Warriors, who doubted themselves, their age, their approach and their confidence after Game 1.

Brown, who scored 13 first-period points, picked up his second foul on a Gary Payton II breakaway layup where officials ruled Brown nudged him from behind. Despite it being just eight minutes into the game, Brown wanted Udoka to challenge the call. It was far too early for that, and Brown was visibly affected by the foul trouble.

Golden State's Draymond Green and Kevon Looney combine for a steal against Jaylen Brown in the third quarter Sunday night.Matthew J. Lee/Globe Staff

Brown finished the game 1 for his final 11 shots, shaken by the officiating, frustrated by Green, who cut under him on a 3-point attempt and then left his large Converse sneakers in Brown’s face after they both fell to the floor. Brown stood up and confronted Green.

But Green had already won. He singlehandedly turned the game ugly, using physical play and trash talk to get into the Celtics’ heads. The Warriors realize they face a talented opponent, one that could easily win the series. Green took it upon himself to turn the series contentious and he got the benefit of favorable officiating when he and Brown weren’t called for double technical on that 3-point foul.

Green already had a tech after an exchange with Grant Williams. The officials knew that and didn’t want to eject him. It’s the type of advantageous calls three-time champions get and the Celtics have to learn how to win despite that.

This series is not going to be completely fair. And Brown was visibly upset with the lack of calls, what Green was able to get away with and the Celtics lack of a true response. He should be angry. The team should be angry because they blew a chance to take command of the series, allowing the Warriors to use their offensive dominance along with some sly tricks to even the series.

“We try not to get too much involved with the officiating,” Brown said. “But obviously on their home floor, so some home cooking, so we’ve just got to come out and raise our level of intensity. They raised theirs, and we kind of was looking around expecting for something to — for somebody to kind of bail us out, and on their home floor that’s not going to happen. We’ve got to raise ours and really no excuses about it, we’ve just got to be better.”

There are no excuses for this one. The Celtics got hustled by the old man at the park. They lost this game mentally, and then their execution deteriorated. It’s a harsh lesson learned but perhaps worth the education if the Celtics don’t let this happen again.