Before Rajon Rondo lost his cool, Brandon Bass and Kevin Garnett couldn't wrestle the ball loose from Josh Smith. (AP)

The Celtics fell 83-74 on Sunday night in Game 1 of their first-round matchup with the Hawks. Rajon Rondo led the way for Boston with 20 points, but was ejected after arguing a call and bumping into official Marc Davis. Kevin Garnett started poorly, but finished with 20 points (8-19 FG) and 12 rebounds.

For the Hawks, Josh Smith scored 22 points and grabbed 18 rebounds.

WHAT WENT WRONG

Hit in the mouth: The Hawks shot 12-of-22 (54.5%) from the field in the first quarter. Atlanta got to the foul line, created easy transition buckets, and sustained the fervent pace that clearly gives it an advantage over the older Celtics.

The excitement and momentum of playing in front of their home crowd certainly didn’t hurt the strong start, and the Celtics rust from resting their starters didn’t help. Either way, Boston couldn’t stop the bleeding early. The C’s never got to the line and missed all seven of their 3-point attempts in the first half (they finished 0-of-11 from beyond the arc).

Tickets sold out: Garnett’s poor first half played a massive role in the team falling behind by double digits. The Big Ticket was aggressive early, which normally translates to good things for the Celtics. Sunday night, however, KG shot 1-of-9 from the field in the first half. Call it a product of rust, but it would have been wise for Garnett to position himself deeper in the paint when his jumper wasn’t falling.

Meanwhile, Garnett’s adversary, Smith, scored 15 points (7-12 FG) and grabbed 11 boards in the first half alone.

Ray of Light: Game 1 went sour rather quickly for the C’s. While that’s certainly concerning,  a bigger issue is the health of Ray Allen. Last Wednesday, Doc Rivers pronounced Allen probable to play in Sunday night’s series opener, by Saturday the prognosis changed to doubtful, and when it came time to tip off the playoff run Allen was in a suit and tie.

The Celtics could very well survive this series against Atlanta and maybe beat the Derrick Rose-less Bulls without Allen, but Allen makes both of those feats easier.

Losing your cool: The Celtics undoubtedly will be holding their collective breath while awaiting the league’s ruling on Rondo bumping the official. There is a chance Boston could be without him for Game 2.

WHAT WENT RIGHT

Taking over: Rondo has been extremely passive in terms of looking for his own shot this season. When the rest of his team struggled in the first half Sunday, Rondo called his own number, scoring 12 points (6-9 FG) and dishing out four assists. He was assertive because he needed to be. Rondo finished with 20 points, 11 assists and four steals.

It’s frightening to think how ugly this game could have gotten if not for Rondo’s performance. All year long the Big Three have stated this is Rondo’s team, and Game 1 was proof of that.

Getting off the mat: Several times — especially in the opening stages of the second half — the C’s looked primed to make a run and cut the deficit to single digits. The Hawks did a great job thwarting those attempts, and it wasn’t until late in the fourth quarter that Boston closed the gap to five following a 21-7 run. The Celtics then dwindled the advantage down to as little as four with just over a minute left, but Rondo’s subsequent ejection was the final undoing for Boston.

It would have been easy for the Celtics to call it a night, especially since the deficit floated between 10 and 15 points for the better part of the game. Boston struggled, sure, but its effort never relented. The C’s rust was palpable: They shot 39 percent from the field, yet were still in contention late in the contest. If the Celtics can play with the same mettle the rest of the series, their desired results should follow.