ATLANTA — When the Boston Celtics emerged from their timeout with about nine minutes remaining in Sunday’s second quarter against the Atlanta Hawks, some started looking around for one of their star teammates.

They wondered: Where was Jaylen Brown? Did he disappear?

No, he didn’t. But he did change face.

At the very moment his teammates were walking onto the floor to resume play, the Celtics’ electric scorer was indeed right there with them. He was just sporting a slightly different look.

Brown had ditched the black face mask he had been wearing in games since the middle of February. The Atlanta-area native’s old visage had finally come back into focus, and with it arrived a major and timely change.

“I was talking to [Marcus] Smart when JB took his mask off and was like, ‘Oh, s—. It’s go time,'” Celtics guard Jayson Tatum said. “I knew he was going to turn it up a notch.”

Across the first 15 minutes of Game 4 of the Celtics’ Eastern Conference opening-round series against the Hawks, a mask-wearing Brown had been held to a meager two points on a 1-for-7 shooting performance from the field.

Because of that abysmal offensive showing, Brown felt something needed to change.

“Maybe it was all in my head, but I just needed a different look,” he said. “As soon as I took it off, things started to turn around a little bit.”

Things turned 180 degrees.

From the moment Brown took off the mask — which he started wearing days after sustaining a fracture along his cheekbone in a Feb. 8 home win over the Philadelphia 76ers — he started playing differently against the Hawks.

Without the mask on Sunday, Brown scored 29 points on an 11 for 15 showing from the field. Each of his three assists and three made 3s came in that stretch in which his face was visible. Brown finished with 31 points, tying Tatum for the team scoring lead in the 129-121 win.

“He’s sitting in the corner, and he gets the ball and then obviously he makes those plays where he’s driving with force, and he throws it off the glass at times, and he makes some great plays,” Smart said. “So we just knew at that moment he was a different JB, and he was going to carry us and bring us home.”

Brown’s performance played a key role in the Celtics returning to Boston for Game 5 on Tuesday with a 3-1 series lead.

Asked what he saw from Brown after his mask came off, Celtics coach Joe Mazzulla quipped, deadpan: “His face.”

“After that, I saw just his poise,” Mazzulla continued, after laughter during the news conference subsided. “And I thought he did a great job making plays at the rim, operating in space, playing off two feet, making the right play. To me, he showed just tremendous poise on the offensive end, and he had the ball in his hands, making plays for himself and others.

“Anytime he got in the paint, he made the right play. So I was really proud of him for that.”

Despite the midgame switch, Brown is likely to remain masked in games going forward. The cartilage around his cheekbone is still healing, he said.

Following the injury, Brown missed Boston’s next four games. He debuted the mask Feb. 19 at the All-Star Game in Salt Lake City, scoring 13 points off the bench and even throwing down an emphatic one-handed dunk in transition.

“I’ve just started getting comfortable with it since I’ve been wearing it,” Brown said Sunday. “When I first put it on, I didn’t like it too much, and I had to wear it.

“[On Sunday], I just needed something different. I don’t know what it was: Change my shoes, wipe my hands off, take the mask off, whatever it is I needed to do, I needed to do to help our team get a win, and that’s what I was trying to do.”