By Matthew Geagan, CBS Boston

BOSTON (CBS) — When Giannis Antetokounmpo followed up his emphatic third-quarter dunk in Game 4 with a glare at Al Horford, it looked like the Celtics were in trouble. The Bucks were cruising and their best player was doing anything and everything he wanted.

Boy did that one backfire on Giannis and Milwaukee.

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Horford was shaking his head as he walked away from Giannis’ mean mug and filed it away in the back of his mind. He took it personal.

The dunk put the Bucks on top 54-48 in the third quarter, and with the Celtics getting bullied and their star players struggling, it looked as though they’d be heading back to Boston in a 3-1 series hole. But Al had other plans.

That glare from Antetokounmpo awoke the beast inside the 35-year-old Horford, who put in a vintage performance to lead Boston to a 116-108 comeback win. Horford scored a new postseason career-high 30 points in the victory, including 16 in the fourth quarter, when he got some revenge on Antetokounmpo with an emphatic jam of his own.

Monday night’s Game 4 win will thus be known as “The Al Horford Game” in Celtics lore.

The Celtics outscored the Bucks 68-54 after the Giannis stare-down, with Horford leading the charge. The fourth quarter was all Boston, with the C’s beating the Bucks 43-28 in the frame to take the game and swing momentum in the series, which is now tied 2-2.

Antetokounmpo was hit with a taunting technical for his antics toward Horford following his dunk. That just provided Horford and his teammates with more fuel the rest of the way.

“I don’t really know what he said, but the way he looked at me and was going about it, didn’t sit well with me,” Horford recalled after the win. “At that point, something switched with me.”

Horford scored 10 of Boston’s first 14 points in the fourth quarter, including a dunk where he blew by Giannis for a nice and-1 jam. Horford is usually as calm as they come, but even he couldn’t contain his excitement in the aftermath of the play.

Horford was 5-for-7 from downtown on Monday night, which forced Antetokounmpo to race out to the three-point line before Horford blew by him for the hoop.

“I got the pass and [Giannis] closed out. I had been shooting that a lot and just figured I’d drive it. Took it in, big play, obviously. Very emotional one,” he said of the jam. “For our group, I think it really got us going even more. ”

Horford said that he picks his spots to let the emotions fly, and his fourth-quarter dunk certainly warranted his explosion. The dunk and free throw tied the game at 81-81 (Horford was hit with a technical for hitting Giannis with an inadvertent elbow), and got everyone in green going, too.

“That was a hell of a play. Any time Al turns back the clock and looks like his old self with one of those dunks, it gets everybody off the bench,” said Jayson Tatum, who scored 14 of his 30 points in the fourth. “Al plays with so much passion and when he plays like that, everyone else has to follow.”

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“We felt it. Everybody did,” said Marcus Smart. “The energy changed for everyone after that and it got him going and it got everyone else going.”

Smart called Horford the best veteran he’s ever played with, one who provides the calming presence that the Celtics need when things aren’t going their way.

“It never changes with him; whether it’s going good or bad, he’s going to be him,” said Smart. “Nine times out of ten it’s going to work in our favor. He came through big for us and helped secure the win for us tonight.”

There was still work to be done after Horford’s slam. After Antetokounmpo put Milwaukee on top 94-92 with one of his many driving layups, Horford answered on the other end with a three, giving Boston a 95-94 lead with 5:40 to play. He then corralled the rebound on a Brook Lopez miss, and finished with a nice hook shot — and the foul — on the other end. That three-point play was Horford’s final bucket of the game, putting the Celtics up 98-94.

Horford played 42 minutes on Monday night, hitting 11 of his 14 shots. He pulled down eight rebounds and handed out three assists in the win.

Boston was down double digits in the second half and trailed by seven heading into the fourth. The Celtics were without Robert Williams (knee soreness) and Jaylen Brown picked up his fifth foul late in the third quarter. All of this was against the defending NBA champs on the road.

The Celtics came back Monday night thanks to Al Horford. They don’t win the game without him, and it wasn’t just the hoops that he made. He played solid defense on Antetokounmpo, making him work whenever the two tangled bodies. And as Horford was going off in the fourth, Giannis was held to just six points, mostly thanks to Al’s defense.

“Al carried us for that second half and just ignited everyone else to get it going,” said Tatum.

“No one can ever say anything bad about Al,” Tatum added. “He’s helped me a great deal in our three years together, and I’m very lucky and fortunate to call Al a teammate.”

“Al, man. He’s been doing this for a very long time and he understands what he brings to this game and to this team,” said Smart. “We need every little bit of it ever night. It’s a big, big, big, big key, having Al with us. He makes us that much better.”

It’s incredible to think back to the summer, when Brad Stevens acquired Horford from the Oklahoma City Thunder for Kemba Walker and a first-round pick. It was seen as two teams trading unwanted players on unwanted contracts. Horford barely played for the Thunder last season after the 76ers dumped him on OKC.

Maybe that explains why he has so much juice this season for Boston. But one thing is clear whenever Horford talks about coming back to Boston; the man loves being a member of the Celtics.

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He’s hungry to win with this group, and he’s more than happy to be the veteran of the bunch. But he’s a savvy vet at that, and on Monday night, he proved that he can turn back the clock for a dominating performance. It couldn’t have come at a better time for the Celtics.