BOSTON -- After watching the Celtics fall behind by more than 20 points for the third time in the past five games in Sunday's 130-124 loss to the Miami Heat, Celtics coach Brad Stevens said it's on him to figure out how to get his team off to better starts.

"Everybody knew the circumstances, right? Everybody knows where we stand," said Stevens, whose team is in seventh place in the Eastern Conference standings, two games behind Miami and Atlanta. "They know it, we know it, and they outplayed us. You've got to give them credit for that.

"I need to do a better job of making sure the first half doesn't get out of hand. If it's at the start or if it's in the middle or wherever. And one more thing, they're really good. So I also recognize that. I recognize that we have our hands full in this matchup, regardless of everything. But we have to play better in that first half to give ourselves a chance."

The Celtics, who trailed by 20 in the second half Friday in Chicago before a spirited comeback fell short, have lost three of their past four games -- and would have lost four of five if they hadn't staged an incredible comeback from down 32 to beat San Antonio on April 30 at the Garden. Boston has fallen behind by at least 20 points 13 times this season.

As a result of Sunday's loss, the Celtics sunk to seventh in the East, and if the Celtics lose again to the Heat in the rematch between these two teams on Tuesday, they will be virtually assured of being in the play-in tournament, which would likely begin with the Celtics hosting the Charlotte Hornets in the 7-8 matchup.

Of more immediate importance to Boston, however, is figuring out what is causing this team to get off to such poor starts on a consistent basis. That was certainly the case Sunday, as Miami jumped out to a 26-point halftime lead, shooting 65 percent from the field and going 11-for-20 from 3-point range.

"That has to come from within, from each one of us," said Evan Fournier, who had 30 points as he continues to play well after several off games in the immediate aftermath of his return from contracting COVID-19.

"There's really not much to say about it, to be honest. We just have to start individually better and do whatever to get us going. A 1 o'clock game is not an excuse at all. We just have to be better individually, period."

In each of their recent slow starts, the Celtics have staged big second-half comebacks. But those have only masked Boston's issues during its slump -- including repeated struggles defensively.

After finishing in the top six defensively each of the past three seasons, the Celtics are ranked 18th after Sunday's loss.

"I do think that ultimately, we've done a better job in the paint, most of the time, although there were times tonight [that we didn't]," Stevens said. "But we're slow to react to the ball moving and we're not quite on a string at the way that a small team needs to be. Because last year, we were a little bit bigger, but we were better with our ball pressure and quicker into air space of shooters. This year, we're a step slow."

Boston certainly looked a step slow against the Heat, now winners of seven of their past 10 games and still hoping to get into a 4-5 series against either the Atlanta Hawks or New York Knicks in the first round. (The alternative: finishing sixth and playing either the Milwaukee Bucks or Brooklyn Nets, or falling into the play-in tournament in seventh.)

The Celtics, meanwhile, say they aren't focusing on finding a way to climb the standings. They're looking for a way to right themselves over these final few days of the regular season before heading into whatever part of the postseason awaits them.

"I think obviously we know how to respond," Jayson Tatum said. "We've shown that in many games. I guess the answer is it's a choice. I don't think that we're necessarily good enough to just turn the switch on, like all right, let's play. Certain games, here and there, we've done that, and been able to win.

"Obviously today we dug ourselves a hole. We gave ourselves a chance. We're always going to do that. But I know that we can do better, do more, especially in the beginning of the games, just play with a little bit more toughness and play faster. When we get stops in the second half we play fast and play to our advantage, and we look pretty good. So we've just got to start like that."