Boston’s Robert Williams III, left, and Miami’s Duncan Robinson wait for a rebound on Sunday in Boston. The teams meet again Tuesday. Steven Senne/Associated Press

BOSTON — The Boston Celtics watched their playoff hopes slip drastically on Sunday, but it’s not yet a doomsday scenario.

The Celtics lost 130-124 to the Miami Heat in Boston with a subpar performance and another slow start put on display with a 26-point deficit at halftime. Now comes the most important game of their season with Tuesday’s rematch against the Heat.

Boston’s hopes for the No. 6 seed stay alive with a win, while with a loss it is all but guaranteed an appearance in the play-in tournament. Thehe Celtics and Heat are 1-1 ghead-to-head, so Thursday’s game is also for the always-important tiebreaker.

But Boston needs to be better than it was on Sunday.

“When we’re not completely engaged, for whatever reason, we’re just not good,” Celtics Coach Brad Stevens said. “When we’re completely engaged, we’re a good basketball team. When we’re not, we’re not even close to being able to compete with anybody, let alone a team like Miami.”

The top six seeds will not play until the quarterfinals, and thus avoid this year’s new playoff wrinkle – the play-in tournament involving seeds 7 through 10. Boston stands at 35-33 entering Monday, two games behind the Heat and Hawks and two games ahead of eighth-place Charlotte. Atlanta and Miami have the same record, but the odds heavily favor the Hawks finishing ahead of the Heat because Atlanta owns the tiebreaker and has the far easier remaining schedule.

The Wizards, three games behind Boston, jumped past the Pacers by beating them Saturday night. Washington (32-36) has won 13 of 16 games and has the season tiebreaker over the Pacers (31-36), but Bradley Beal is expected to miss at least two games with a balky hamstring for the Wizards.’

There’s still an outside chance the Wizards can catch the Hornets, but Charlotte is fighting to maintain position or even potentially pass the Celtics. Boston does have the tiebreaker over Charlotte.

For Sunday’s game against Miami, Boston did not have Jaylen Brown (ankle injury). But Boston announced Monday that Brown also has a significant wrist injury and will miss the rest of the season. He is averaging 24.7 points per game.

So if Boston wins Tuesday, the Celtics would jump Miami if Miami loses two of its final three with a difficult schedule and Boston wins two of its final three with a relatively easy schedule (at Cleveland on Wednesday, at Minnesota on Saturday and at New York on Sunday).

The teams that finish seventh and eighth in each conference will play May 18 on TNT, with the winner clinching the seventh seed and the loser meeting the winner of the 9-10 play-in matchup for the eighth seed.

The Celtics spent Sunday’s postgame session bemoaning how the Heat sliced them up defensively, with Miami’s 79 first half points the second most in franchise history.

“Our defense isn’t as good as it’s been in the past,” Stevens said. “We’re slow to react to the ball moving. Because last year, we were a little bit bigger, but we were better with our ball pressure and quicker into the air space of shooters. This year, we’re a step slow.”

STARTING CENTER Rob Williams, who has been battling turf toe, felt good enough to start and play 11 first-half minutes on Sunday against Miami. But the painful ailment became too much to play through and the Celtics’ ruled him out for the second half. He’s likely headed for a permanent spot on the team’s injury report for the remainder of the season.

“Too uncomfortable to keep going,” Stevens said. “I don’t know what’s going to (happen). It could be game-time decisions every day. We may have to sit him. I don’t know what the deal is right now.”

After trading Daniel Theis at the deadline and releasing Mortiz Wagner, Boston has limited available options at center. Tristan Thompson, Luke Kornet and Tacko Fall are the team’s remaining true centers. Stevens has used Kornet sparingly and Fall rarely at all.

Grant Williams and Jabari Parker can play the five in small lineups. But Williams played eight minutes and Parker didn’t check in.

“That’s something we’re going to have to manage, Stevens said. “I played Tristan like 15 straight minutes in the second half.”

EVAN FOURNIER’S terrific May continued against Miami as he led the Celtics in scoring (30 points) and assists (eight). He’s averaging 21.5 points per game this month.

“The more I play, the more familiar I’m going to be with the offense and the defense. That was the main reason I absolutely wanted to come back as soon as possible from COVID,” Fournier said. “I’m getting more comfortable out there. I’m going to do a lot better and I have to do more.”