Boston Jaylen Brown heads to the locker room after the Celtics lost to the Atlanta Hawks in Game 5 of their first-round series on Tuesday. Charles Krupa/Associated Press

BOSTON — The Boston Celtics were six mere minutes away from the perfect ending unfolding on Tuesday night at TD Garden.

Three days off loomed before a second-round matchup against the 76ers if Boston was able to take care of business against a short-handed Atlanta Hawks squad playing without a suspended Dejounte Murray. A Game 5 victory would have also cut down on potential recovery time for Philadelphia’s Joel Embiid with his sprained knee as well, forcing the MVP into action on Saturday, a potentially big edge in the heavyweight tilt.

The Celtics led 111-99 with 5:15 left in regulation before an unprecedented collapse turned a dream scenario into a nightmare as the Hawks closed the game out on a 20-6 run in the final five minutes to escape with the win. Trae Young scored the game’s final 14 points for Atlanta while Boston’s offense, which had scored with ease for the past three games, self-combusted when it mattered most. What happened and how exactly did it happen in Game 5? A perfect storm of mistakes and missteps breathed life into Atlanta.

BIZARRE BLAKE GRIFFIN APPEARANCE

The TD Garden crowd loved to see the veteran big man enter the game with 10:24 left in the game for Sam Hauser in an eight-point contest. It was the first appearance of the series for Griffin with Coach Joe Mazzulla largely sticking to a eight-man rotation, leaving bigs like Griffin and Grant Williams on the bench.

Griffin’s appearance in this spot made for a fun story but was bizarre situationally as he took the floor with Rob Williams to see a Hawks team that was going smaller with DeAndre Hunter playing power forward at the time. Griffin made a couple of hustle plays that the crowd enjoyed but was targeted on a pair of pick-and-rolls that led to easy layups at the start of Atlanta’s game-changing 12-0 run.

The fact Griffin got six minutes in a surprise stint during a crucial fourth quarter was bizarre. Boston’s offense started to sputter with him in the game and foreshadowed a poor choice by Mazzulla to go with double bigs in his lineup for almost the entire fourth quarter, which opened the door for open uncontested 3s in the pick-and-roll in drop coverage for a team that was playing small and hitting those shots all night.

DERRICK WHITE BENCHED FOR BIG LINEUP CHOICES

One player who scored consistently all night was White (18 points on 7-of-11 shooting from the field) and amid a strong game, good things have also happened all year long when he was on the floor in crunch time. Still, as Boston’s offense sputtered late, Mazzulla opted to go with Marcus Smart over White (and Malcolm Brogdon) with three minutes left and the Celtics winning 111-105. Young proceeded to hit two straight 3s after White hit the bench while Smart piled up gaffes including an ugly pass that led to a turnover, a moving screen and an ill-advised loose ball foul that put the Hawks ahead with 15.1 seconds left.

While the Celtics’ defense had some bad letdowns due to the clean looks they gave Young in switches against their bigs, the team offensively also let the Hawks off the hook by not playing some of their best shot creation weapons (Brogdon, White) against the Hawks’ weaker defenders when it mattered most. With Jayson Tatum struggling through a dud of a quarter offensively, alternatives better than Smart were available to win the game with offense. However, Brogdon didn’t play for the final seven minutes, and White sat for a key three-minute stretch in which Atlanta took the lead with 1:39 left.

SLOPPY TURNOVERS

Boston shot 50 percent in the fourth quarter but their five turnovers gave Atlanta 10 points that proved to be the difference on a night they were hitting from all over the floor. Four of those turnovers came in the final three minutes and featured bad decisions from everyone (bad passes from Tatum, Smart and Jaylen Brown in key spots) that kept the hosts from putting the Hawks away. The lineup decisions are worth the second guess but there’s no doubt that the Celtics’ cavalier passing choices fueled a horrific collapse here. They deserve responsibility for that.

TIMEOUT MISMANAGEMENT?

Mazzulla’s timeout usage questions had largely gone away in recent weeks but deserve a second look after this collapse. With the offense floundering, he declined to call a use it or lose it timeout in the midst of a Hawks’ 9-0 run before the three-minute mark in regulation. Smart turned the ball over on that possession, leading to a game-tying Young 3.

The Celtics also gave away precious seconds with 15.1 seconds left after Young took the lead with free throws. Boston simply dribbled the ball up the floor while trailing and burned six seconds before Atlanta gave a foul, shrinking their margin for error in the closing seconds.

White eventually hit free throws to put the Celtics back ahead with 7.1 seconds left but Boston could have had more time at the end of the game if they had drawn up that play a few seconds earlier with a timeout. The domino effect from there (Young shoots his 3 sooner and gives Celtics more than 1.8 seconds left on clock to tie the game at the end of regulation) could have boded large. Instead, Boston came up empty on its final possession with just two seconds left and no margin for error.

The Celtics won’t have much time to stew on this one before Game 6 in Atlanta on Thursday night but this was a team collapse in every sense of the word. Poor choices by the players and the coach gave away a crucial opportunity and made things much harder for this squad than they needed to be.