BOSTON — There was a sense of reality as the Celtics embarked on a season without title expectations for the first time in a few years. Jayson Tatum was going to miss most of the season with his Achilles injury. A gap year was understandable as the C’s also shed salary off their payroll last summer.

Despite all that, the Celtics’ pitch this season was simple: They were going to play hard . They were going to be fun to watch. They were going to mix top-end talent with hungry young players looking for their spot in the league. And, for the most part, they’ve delivered on that to open the season.

That pitch fell apart Friday. The Celtics lost 113-105 to the Nets , an East cellar dweller, in a listless, disappointing night. The Celtics can’t have that. The loss was a reminder that, when

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