Atlanta’s NBA Cup victory at TD Garden on Nov. 12 proved costly for the Hawks.

The NBA on Tuesday fined the Hawks $100,000 for sitting All-Star point guard Trae Young for the tournament matchup against the Boston Celtics.

The Hawks ruled out a total of six players for that game, including Young, who was listed on their injury report with right Achilles tendonitis. But an NBA investigation ruled that Young — Atlanta’s leader in points and assists — was healthy enough to play and should have been active.

“Following an investigation, including review by an independent physician, the NBA determined that the Hawks held Young out of a game that he could have played in under the medical standard of the (Player Participation) Policy,” the league said in a statement. “The organization’s conduct violated the Policy, which is intended to promote participation in the NBA’s regular season.”

Per the terms of the Player Participation Policy, which was introduced last season, a “star player” missing a nationally televised or NBA Cup game automatically triggers a league investigation into his absence. A star is defined as anyone who made an All-Star or All-NBA team within the last three seasons — the cohort of players who draw the most fan and media attention and are central to the league’s marketing efforts.

The NBA evidently determined Young did not have a “bona fide injury,” which would have excused his absence.

Young was active for Atlanta’s other 17 games this season, averaging 21.9 points and a league-high 11.9 assists per contest. The Hawks didn’t need him to secure a 117-116 win over Boston, relying on the likes of Dyson Daniels, Jalen Johnson and Onyeka Okongwu to upset the defending NBA champions on their home floor.

Celtics head coach Joe Mazzulla said his team was “outplayed (in) every facet of the game” in that loss, which complicated Boston’s path to the NBA Cup knockout rounds.

The Celtics and Hawks both will close out group play this Friday night (against Chicago and Cleveland, respectively), with the Group C winner still undecided. Boston needs a win and an Atlanta loss to guarantee safe passage to the quarterfinals.

Mazzulla’s squad also could advance if the Celtics and Hawks both win, but their fate in that scenario would hinge on point-differential tiebreakers.