Celtics center Robert Williams III had 13 points and 15 rebounds in Boston’s 129-121 Game 4 victory over the Hawks on Sunday in Atlanta. Brynn Anderson/Associated Press

The Celtics’ depth is a luxury for Coach Joe Mazzulla but it also invites plenty of tough decisions. Mazzulla faced some of those decision in the final minutes of Boston’s 129-121 Game 4 win over the Atlanta Hawks on Sunday night.

The Celtics were keeping a scrappy Hawks team within arm’s length for much of the fourth quarter but the hosts were not going down quietly. With crunch time approaching and several of Mazzulla’s closing options playing well (Malcolm Brogdon, Derrick White, Marcus Smart, Rob Williams III) a tough decision would need to be made on who to roll with to help put the series away.

Given the personnel against Atlanta, Mazzulla went with the grouping that he completely abandoned in a Game 3 loss: Double bigs with Al Horford and Williams after Horford entered the game for Brogdon with 5:30 remaining.

There was no clear indicator for why Mazzulla would roll with this group. Williams was an easy choice to finish with given his standout performance (13 points, 15 rebounds, 2 blocks) but Horford (0 points) hadn’t done anything offensively all night. Together, the duo had played just 19 minutes together in the first three games of the series (none at all in Game 3) and had a minus-11.1 net rating.

Yet Mazzulla’s feel to go back to his old starting five was dead on (he went with Smart over White with just over three minutes left) in this instance as this duo helped Boston secure an impressive Game 4 win.

Horford started with the impact plays late, drawing a sixth foul on Bogdan Bogdanovic with five minutes remaining while going for an offensive rebound with his size advantage. One of Atlanta’s top offensive weapons was eliminated and Hawks Coach Quinn Snyder was forced to turn back to John Collins who struggled throughout Game 4 (1 of 9 late).

From there, Williams capped off his big night with a pair of crucial layups after Jaylen Brown made some great reads at the rim to find the big man. Williams was able to easily use his size to finish against smaller rotation defenders and ensure Boston got great looks to help keep a late Hawks run at bay after the team closed within five points on two occasions.

Ultimately, the Celtics had a plus-25 net rating when they rolled with Williams/Horford in Game 4 for 16 minutes. In the fourth quarter, the duo had a plus-20 net rating to help close the door on Atlanta.

“We definitely played them together a little bit more,” Mazzulla told reporters in Atlanta of the double bigs. “They were both tremendous tonight. I thought Al only got two shot attempts but he was a plus-17 and everything he gave us was instrumental. When those two guys are connected and playing really well, it really helps our physicality.”

Williams got his first crunch time minutes of the series in the matchup and appreciated the faith his coach saw him, even going to him with his five fouls.

“I mean, he trusted me,” Williams said. “That’s the biggest thing. He trusted me. My teammates trusted me. They help me not be in a position where I have to foul, so I could finish.”

Boston’s original starting five struggled for much of the regular season but Mazzulla finally found his way back to it in Game 4 in a critical spot. The Celtics’ coach knows this team has a higher ceiling with Williams in the fold and he found it at a critical spot.

“(Williams) just has to be consistent,” Mazzulla said. “Then he plays at that level, we’re a different team. He brings an energy and he brings a level of joy to our team constantly. When he plays like that, we’re different.”

Mazzulla is going to be tested by far tougher teams in the rounds to come but his decision making in Game 4 was a great indicator that he’s up to the challenge of his first postseason run. Boston likely shut the door on a Hawks team that refused to lay down for them with Mazzulla pushing the right buttons along the way.