After a five-point performance on Sunday that he termed “unacceptable,” Arron Afflalo was back in the gym on Monday working overtime on his shooting.

The Knicks’ loss to Golden State on Sunday and his own poor play weighed on Afflalo going into Tuesday night’s game against the Celtics. The veteran guard was determined to get off to good start and help the Knicks to what he called “a much-needed win.”

Afflalo was able to achieve one of his goals. He hit his first three shots as the Knicks jumped out to a 9-0 lead.

Afflalo was unable to build much on his fast start, however, and he finished with 18 points on 7 of 16 shooting as the Knicks lost to Boston, 97-89, at the Garden.

The Knicks have dropped six of their last seven. That stretch has coincided with a shooting slump from Afflalo.

“It’s just a long season,” coach Derek Fisher said. “Aaron’s been in the league long enough to know that there are always highs and lows in an 82-game schedule. No matter how hard you’re trying sometimes the ball just won’t go in . . . I think Aaron will be fine.

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“He’s more concerned about us winning games than his individual game. I think as long as that’s his mentality he’ll work his way through it.”

Afflalo made just two of 12 attempts in Sunday’s 116-95 loss to defending NBA champion Golden State. It was supposed to be a test game to see how the Knicks stacked up against the league’s best.

“It’s hard to gauge one game because you don’t know if you put forth your best effort out there,” Afflalo said. “At least I know for me it wasn’t that . . . We’ll see that team again in March and hopefully be more competitive.”

Boston isn’t anybody’s idea of a championship contender, but there are similarities between how the Celtics and Warriors play with a fast pace and small lineups.

“They’re similar,” Afflalo said. “As you can see, the league is kind of transitioning that way a little bit.”

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Afflalo scored 11 of his points Tuesday night in the first quarter as the Knicks took a 22-21 lead. His production went down from there: two points in the second, none in the third and five in the fourth.

In his scoreless third quarter — during which the Knicks were outscored 25-18 — Afflalo had four turnovers, including three in one dizzying 24-second stretch.

“Obviously, I’ll take credit for the turnovers for that stretch,” Afflalo said. “But I think overall defensively we’ve slipped as a team. It’s not even worth talking about. We just have to improve.”

In other words . . . back to the gym.