The Celtics made three critical errors on their final two possessions, and that’s why they lost Game 4

Marcus Smart (left) stood his ground against 76ers center Joel Embiid after getting called for a foul in the first quarter.Barry Chin/Globe Staff

PHILADELPHIA — The Celtics make themselves easy to second-guess because their late-game execution can be questionable. Boston has four playoff losses and three have come because of go-ahead buckets in the final 20 seconds.

Each time the Celtics had a chance to tie or win on their final possession but didn’t even muster a free throw.

Boston could regret how it played in the final 20 seconds Sunday in Game 4 of the Eastern Conference semifinal series against the 76ers. Not only did the Celtics blow a 5-point lead in the final two minutes of regulation, but they made three critical errors in the final two possessions in an excruciating 116-115 overtime loss.

If any of those mistakes wasn’t made, the Celtics win. But now they find themselves tied in a series they could have easily swept, with Philadelphia reinvigorated going back to Boston for Tuesday’s Game 5.

With the Celtics holding a 2-point lead with 26.4 seconds left in overtime the 76ers had 12 seconds left on the shot clock. Joel Embiid got the ball in the paint and attacked the rim. Jaylen Brown left James Harden, who had already hit five 3-pointers, alone in the corner. Embiid dribbled toward Jayson Tatum, sucked in Brown on help, then flipped the ball to Harden in front of the 76ers’ bench. Swish. Philadelphia was up by 1.

“I take full accountability,” Brown said of leaving Harden alone. “It was a bad read. It was my fault.”

With 19 seconds left and two timeouts, and we all know how Celtics coach Joe Mazzulla savors timeouts like silver dollars, the Celtics opted to not stop play and advance the ball.

Marcus Smart dribbled the ball slowly up the floor and lobbed it to Tatum with less than eight seconds left. Tatum needed to go fast. The 76ers were out of fouls. He’s one of the more prolific scorers in the NBA. This was his time to shine.

Instead, Tatum called for a Derrick White screen on Tobias Harris so he could attack the smaller Tyrese Maxey. With three seconds left, Tatum drove to the left, but Embiid approached and Tatum found Smart on the elbow for an open 3-pointer.

It was the right read; the Celtics preach ball movement to the open man, but it was far too late. Smart and Tatum both took too long. Smart’s rushed 3-pointer went in, but after the buzzer, adding insult to poor execution.

“I waited a second too late,” Tatum acknowledged. “When Embiid came over, I tried to kick it out but I probably should have went (accelerated) a dribble or two earlier. I was just trying to make the right play, find the open man.”

If Mazzulla’s going to pass on a timeout and let his players win the game, they have to devise a play better than a late Tatum drive and last-second kick to Smart. It played right into Philadelphia’s hands, just like that inbounds play to Smart in Game 1 when he freelanced and threw an errant last-second pass to Tatum.

This is happening far too often. The Celtics blew Game 5 against the Hawks and have blown two games in this series. Mazzulla previously changed his propensity for passing on timeouts and watching his team adjust on the fly, but he went back to that flawed strategy Sunday.

How many times does he have to watch his players panic or hesitate in late-game situations when they need a bucket? How about that Miami game in January when down 1, Mazzulla let Tatum dribble the ball slowly up the floor with 20 seconds left, and when Tatum was doubled, he threw a bad pass in the direction of Grant Williams for a turnover? They lost.

Mazzulla has to trust his play calls against a set defense will be more effective than running a freelance play in a critical moment.

The Celtics didn’t get off a shot when they had the ball for the final 19 seconds. That’s inexcusable and it could cost them this series.

James Harden and the 76ers were able to execute with the game on the line Sunday. The Celtics? Not so much.Tim Nwachukwu/Getty

Mazzulla was back to his “Joe Cool” personality when asked about the final sequence.

“Jayson made the right play, we just have to play with a little bit more pace,” he said. “Just made the right read, which I thought we did. We got Maxey in the pick-and-roll and he made the right play, just didn’t play fast enough.

“That was a great playoff basketball game. I thought we did a good job sticking with it. We played on the road and a chance to go up 3-1, the game went exactly how I thought it would go and we just didn’t make enough plays down the stretch.”

He was asked about regrets over late-game execution.

“No, it was great execution.”

So Brown admitted fault, Tatum admitted fault, and Mazzulla essentially says, “nothing to see here, we played it perfectly.” Meanwhile, Magic Johnson, who is as congenial on social media as Mr. Rogers, criticized Mazzulla on Twitter for not calling a timeout.

The Celtics played poorly in the first half, allowed Harden to get comfortable offensively, but still played well enough to win and had the 76ers shaken in the fourth quarter. That’s why this was such a painful loss.

Their four playoff defeats are by a combined 15 points. Their six wins are by a combined 88 points. They are by far the best team in the Eastern Conference but one of the worst at closing out games.

Blame Tatum and Smart for waiting too long. Blame Brown for leaving Harden open. But blame Mazzulla for not showing enough confidence in himself and his play-calling to win the game with one final organized offensive set.