LAKE BUENA VISTA, Fla. -- The Boston Celtics needed to win Game 4 of the Eastern Conference finals to even this series at two games apiece.

So why did they -- and, in particular, Jayson Tatum -- come out so flat in the first half, an effort that helped doom them to a 112-109 loss to the Miami Heat and to a 3-1 deficit in this best-of-seven series?

They were at a loss for answers afterward.

"I wasn't aggressive enough," Tatum said. "I didn't score in the first half. That's unacceptable. So I knew I had to play better, and that's what I tried to do."

Tatum finished with 28 points, but none of them came in the first half, when he barely looked to shoot and finished 0-for-6 from the field, including 0-for-4 from 3-point range. He didn't look to have an explosive burst at any point and would either immediately give the ball up upon getting it or, after taking one dribble inside the 3-point line, would instantly try to pass it to someone else.

But for as passive as Tatum was early, he suddenly kick-started his game late in the third quarter with back-to-back and-1 drives to the rim on Heat forward Jae Crowder. He then briefly helped Boston regain the lead early in the fourth quarter before one of several Tyler Herro scoring bursts in this game put Miami in front for good.

"It's basketball," Celtics coach Brad Stevens said when asked why Tatum's first half was so far off his usual standard. "I don't want to over simplify it, but we've all had nights where you don't feel like it's going for you. Then you go into the break -- or you've seen great players do that, at least -- where they go into the break and they can just reset, see one go through the net, and then they go."

"I think that's all Jayson needed to see. He was way more assertive, way more aggressive in the second half. But it was probably because he felt a lot better about it. He was, for whatever reason, I thought in the first half just struggling to find the net, and then it only takes a great player once or twice to get their confidence rolling."

But like so many things about Game 4, the Celtics simply got rolling too late. Stevens admitted Boston was fortunate to be trailing by only six at halftime, and it felt like the Celtics were trying to roll a boulder uphill all throughout the game. Some of that could be attributed to Herro's shooting performance in which he scored 37 points and went 9-for-10 on contested shots.

Some of the blame, however, has to be heaped on Boston's shoulders. The Celtics went 4-for-11 on uncontested looks in the first half, helping dig the hole it had to then attempt to climb out. Boston threw the ball away several times in the fourth, leading to easy Miami baskets at the other end on multiple occasions. They lost the turnover battle 19-8, which turned into 17 points for the Heat.

And, overall, the Celtics didn't play with the same energy they did in the first three games of this series, of which they led in 75% of the minutes but lost two of the games.

Now, after giving away Wednesday's game, Boston finds itself on the verge of being sent home from the bubble with one more loss.

"No, I don't," Tatum said when asked if he could explain what happened to Boston in the first half. "I wish I did. Simple answer is just we've got to be better. We know what's at stake, we know what's on the line, and we've got to play a complete game.

"It's win or go home time. I wish we would have played like that from the start. I take a lot of blame, you know, I didn't play like myself in the first half. So I've got to be better to start the game off on both ends."

The Celtics put themselves in a position where, thanks to dropping the first two games of the series despite controlling most of them, they couldn't afford to have any slippage as this series wore on. But that's exactly what happened in Game 4.

Now, as Boston prepares to return to action here at AdventHealth Arena for Game 5 on Friday night, it can't afford to have that happen again.

"[We] definitely have to stick together," Jaylen Brown said. "Fight to the end. I think that's the best thing that we can do to represent this organization, represent ourselves, represent our families, is stay together, and play hard. And I think everybody will be a part of that result.

"If we don't stay together and we fall apart, it's not going to be a good look for us. Staying together during these moments, and having each other's backs, that's what it's about. I'm looking forward to the film session, I'm looking forward to meeting with our guys, our film coach to watch, and come out next game and fight."