The Hawks were missing more than their head coach.

Playing without Mike Budenholzer, who had to leave the team for a family emergency, the Hawks suffered through a poor second half en route to a 106-93 loss to the Celtics Friday night at TD Garden.

Assistant coach Kenny Atkinson led the Hawks. “This is a pretty close-knit group, obviously close with Bud,” Atkinson said before the game. “It will be interesting to see how they react tonight with all the emotion.”

The Hawks (8-3) started well but couldn’t maintain as the Celtics (4-4) built a 15-point second-half advantage. The Hawks rallied within three points but could get no closer.

Key players

Isaiah Thomas: The Celtics point guard scored a game-high 23 points, 17 in the second half, and had 10 assists and five rebounds.

Jae Crowder: The Celtics forward finished with 13 points on 5 of 14 shooting but scored five straight points during a game-deciding run in the fourth quarter.

Paul Millsap: The Hawks forward finished with 14 points and eight rebounds after scoring 10 first-quarter points.

Five observations

Moose loose

The Hawks rallied in the fourth quarter with a 12-0 run with Dennis Schroder and Mike Muscala doing most of the work. Schroder scored the first seven points and Muscala followed with the next seven. Muscala, who has played sparingly after spraining his right ankle in the season opener, ended a 12-0 Hawks run with his points. The Hawks continued to come back and got as close as three points with 2:50 remaining. However, the Celtics scored the final 10 points.

No doubting Thomas

Thomas turned it on in the third quarter for the Celtics. The 5-foot-9 point guard scored 11 points in the period (on 4 of 8 shooting). He then hit back-to-back corner 3-pointers with seven minutes remaining to end that 12-0 Hawks fourth-quarter run as they pulled within three points, 86-83. The Hawks even defended Thomas with Kent Bazemore in the final minutes.

Too sloppy

The Hawks did not start the third quarter the way they started the first. They played with a lack of energy and were turnover plagued. The Celtics outscored the Hawks 35-24 in the third quarter, including a 21-9 run to end the period. The Hawks committed eight turnovers in the third quarter alone.

Poor second quarter

The Hawks had a five-point lead three times in the second quarter but could never get any separation. In fact, the Celtics erased the deficits and took a brief lead before they teams went into halftime tied at 45. For the second quarter, the Hawks shot 8 of 21 from the field (38 percent).

Much better start

The Hawks have struggled out of the gate in recent games. Not Friday. They shot 11 of 20 from the field (55 percent) while holding the Celtics to 9 of 27 from the field (33 percent).