ATLANTA — The final score was ghastly, yet this game was winnable for the Celtics if they had decided to execute in the early going.

The Hawks missed 28 of their first 34 shots. They looked lethargic and frustrated, but the Celtics could only build a measly 10-point lead that was obliterated in a matter of minutes.

The Celtics were embarrassed Tuesday, 110-83, in Game 5 of the Eastern Conference first- round series, with Kris Humphries canning mid-range jumpers in the second half.

Boston didn’t necessarily give up, but it seemed the Celtics thought they had this game won after the first period. The Celtics refused to place any additional pressure on the Hawks, who began knocking down shots and then couldn’t miss, canning 10 consecutive in a second-quarter stretch as they raced to an 8-point halftime lead.

This was a bizarre game in which the Celtics were playing stellar early defense, and then they simply stopped covering the extra pass and the Hawks played with a confidence not yet seen in this series.

Coach Brad Stevens should be questioned for a couple of his decisions. He did not call a timeout until it was too late during a pivotal 17-1 run that turned a 9-point deficit into a 26-point ditch. And then he continued to play Isaiah Thomas late in the third period when the Hawks were leading by 26, and his guard sprained his left ankle in the fourth, forcing him from the game.

The Celtics have not been a good road team during this series, something they prided themselves on during the regular season. Their defense in the final three quarters Tuesday was putrid. They cooperated fully with Atlanta’s game plan and settled for 3-pointers — and, of course, missed most of them.

Thomas was smothered by Atlanta’s defense, as the Hawks sent two or three defenders at him each time and after he seemed to have that look figured out in Boston, he reverted back to his roaming and passing ways until he left with his injury.

Stevens and his team will have to shake off this defeat with another game Thursday. And no matter what happens in Game 6, they are going to have to win in Atlanta to win this series.

The Hawks began the second quarter having missed 12 of their first 13 shots. And the lead was only a modest 29-19. Atlanta was reeling after losing two games at TD Garden and its offense was sputtering. But the Celtics couldn’t capitalize because no one would take the scoring duties away from Thomas, who was being stymied.

“That was their game plan, they put two or three guys on me every time I touched the ball,’’ Thomas said. “Their game plan was to let the other guys beat [them]. It should be a sign of disrespect to my teammates for them to put two on the ball every time I have it. Other guys have to step up and make plays. That’s what it comes down to.’’

With Avery Bradley out with a strained right hamstring, the Celtics lack a dependable secondary scorer besides Evan Turner, who led Boston with 15 points. Jae Crowder scored 10 but again fell prone to launching 3-pointers. He attempted six of them and didn’t have much of an impact.

Jonas Jerebko took just four shots in 24 minutes and Marcus Smart canned a pair of 3-pointers early and that was about it.

The Celtics can’t be this schizophrenic between TD Garden and Philips Arena. They have to find a way to play more consistently regardless of the venue, and when the Hawks shoot 17.6 percent through the first 18 minutes of a game, they need to put a stranglehold on the outcome.

Thomas made the unusual move of challenging his teammates. Celtics other than Thomas are shooting 38.5 percent from the field and 27.9 percent from long distance.

Atlanta made a statement in Game 5, indicating it doesn’t believe the Celtics can win unless Thomas dominates on offense.

“It’s tough for me because I feel like I can score on anything but as a point guard I’ve got to make the right play and I’ve got to trust my teammates,’’ said Thomas. “I know once my teammates knock down shots, it’s going to open up for me throughout the game. [Tuesday] it didn’t happen.’’

It’s very doubtful the Hawks will again go through an offensive skid as they did to begin Game 5. They almost asked the Celtics to take a 20-point lead, squeeze any enthusiasm from the Philips Arena crowd, and steal the advantage in the series.

Instead, the Celtics were passive, hoping to extend the lead with long 3-point shots. And the moment the Hawks put together a couple of positive possessions and closed in, the Celtics were done. That’s a testament about a team that prides itself on never relenting.

“We were struggling to make shots,’’ Stevens said. “Yeah, there’s a little frustration but it’s a long game. I just felt like if we continue to guard this way we’re going to give ourselves a chance, regardless. But the third quarter, we weren’t very good.’’

The game was lost in the second quarter, when the Celtics blew their chance to take command. That can’t happen again if the Celtics are to have any opportunity to send this series back to Atlanta for Game 7 Saturday.

Gary Washburn can be reached at gwashburn@globe.com. Follow him on Twitter @GwashburnGlobe.