Thunder 91
Celtics 79

OKLAHOMA CITY — This is where the wins come sweeping down the plain, straight into the Oklahoma City Thunder’s win column. The Celtics hoped to steal one Sunday at Chesapeake Energy Arena, hoped to prove that their recent success without Rajon Rondo was not a fluke and that they are, in fact, contenders.

Instead, the Celtics received a dose of reality by swallowing a 91-79 loss in front of a national television audience. Entering the game, Boston had won 14 of 18 since Rondo went down with a season-ending knee injury, the second-best record in the NBA over that span behind Miami (18-2), but it hadn’t faced a test like Sunday: a road game against a young, athletic powerhouse at full strength.

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What the Celtics learned is that their already-microscopic margin of error all but evaporates against such teams, that any mistake — and especially a string of them — can prove fatal.

And they learned that lesson by committing too many turnovers (19), by fouling too much, and therefore being outscored by too much at the free throw line (13 points), and by missing too many second-half shots (30), including several of which were wide open.

Because of all those errors, a win never seemed within the Celtics’ reach or out of the Thunder’s clutches.

Oklahoma City opened the fourth with a game-deciding 11-2 run that Boston chipped at, but could never overcome.

“We just never got a rhythm. Never did,” coach Doc Rivers said after his team’s five-game winning streak was snapped.

If there was an encouraging sign for the Celtics, it was that they could play so poorly in so many key areas and still contend — they trailed, 68-65, entering the fourth quarter — against an elite team such as the Thunder, especially on the road.

“It’s definitely something we can learn from, playing one of the better teams in all the league,” Paul Pierce said. “We can pretty much compete with anybody, but we have to bring our ‘A’ game most of the time and that involves not turning the ball over and not fouling as much.”

Pierce carried the Celtics as best he could, with a team-high 20 points on 6-of-12 shooting, but he received little help. Avery Bradley scored 12 points and Kevin Garnett added 10 points and 11 rebounds, but Garnett, who was hounded by ex-teammate Kendrick Perkins, struggled, shooting 5 of 19 from the floor.

Kevin Durant scored a game-high 23 points and grabbed 11 rebounds, and Russell Westbrook added 15 points as the Thunder improved to 28-4 at home.

The Celtics held the Thunder, who came in averaging a league-best 107 points, to their third-lowest scoring total of the season.

But defense wasn’t what Rivers lamented most after the game. It was the Celtics’ offense, which he said was “bad all game.” Specifically, Rivers said players tried to win the game by themselves and therefore took several less-than-ideal shots.

“Sometimes you want to win too much, too bad, and I thought that was us,” he said. “That’s what good teams make you do sometimes. You want to beat them and you step outside of what you’ve been doing to win games.”

And then there were the free throw issues.

Oklahoma City made 27 of 33 from the charity stripe; Boston made 14 of 20.

“They just crushed us, and that was the exact opposite of what happened in Boston,” Rivers said, alluding to their Nov. 23 matchup at TD Garden, when each team shot 24 free throws.

At one point late in Sunday’s first half, after which the Thunder had made 19 of 21 free throws, compared with 4 of 7 for the Celtics, Rivers came over to the scorer’s table, smiled, and joked, “I don’t think that foul strategy is working too well.”

The Celtics’ 19 turnovers tied for their third-highest total of the season, and the Thunder converted those giveaways into 19 points.

Despite their mistakes, the Celtics were down, 50-45, at halftime. It helped that they shot 51 percent (19 of 37) from the floor in the first half, but that percentage plummeted after intermission, when the Celtics missed 30 of 40 shots.

In the fourth quarter, the Celtics completely unraveled, missing 18 of 22 from the floor.

Rivers called the Celtics’ second-half shot selection “awful” and said the players saw matchups they could take advantage of but that they didn’t share the ball enough.

“We need to get back to trusting,” said Jeff Green, who had 8 points in his first return to Oklahoma City since the Thunder traded him to Boston in 2011. “I thought we rushed [shots]. I know I rushed a lot today. I was overthinking everything.

“We’ve just got to get back to the way we play.”

The Celtics have a cornerstone of principles for how they must win games without Rondo: ball movement, playing up-tempo, earning key defensive stops, a balanced scoring effort. They’ve drifted from these ideas at times, but they’ve been able to recover enough to eke out wins, which is why they had piled up 14 of them in 18 games before Sunday.

In game No. 19, though, the Celtics learned how costly those lapses and mistakes can be, a lesson they’d do well to remember as the playoffs near.