The Boston Celtics have had one of the youngest benches in the NBA this season. Until the trade deadline acquisition of Evan Fournier, and subsequent signing of Jabari Parker, the only long-term veterans Boston had off the bench for most of the year were Tristan Thompson (for just 10 games) and Jeff Teague (for 29 games). Everyone else has four years or less of experience on most nights, including two rookies, two second-year players and a third-year often-injured big man.
That sort of youth has had the Celtics reserves near the bottom of the league in productivity, despite Brad Stevens leaning on them to play league-average minutes.