By Matthew Geagan, CBS Boston

BOSTON (CBS) — Wednesday night wasn’t the worst loss of the Celtics’ 2014-15 campaign, but it was certainly one of the more disappointing.

The Miami Heat came into Boston without the services of Dwyane Wade, Chris Anderson or Hassan Whiteside, less than 24 hours after suffering a buzzer-beating loss to the Milwaukee Bucks.

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The Celtics had the benefit of an off-day on Tuesday, following an impressive win over the Nets in Brooklyn Monday night to right the ship, and with Isaiah Thomas making his return to the floor, they seemed all but certain to improve their postseason credentials on Wednesday night.

Instead, the Celtics gave a lackadaisical effort for three quarters on their way to a disheartening 93-86 loss.

“Sickening. Sickening, man,” forward Jae Crowder said after the game. “That loss hurts. I don’t know what to say about it. We didn’t show up.”

After turning the ball over just 10 times on Monday night, the C’s had that many in the first half of Wednesday night’s tilt, allowing Miami to open a 17-point halftime lead. Boston finished the evening with 17 team turnovers, gifting 18 points to the Heat.

Thomas looked rusty in his return, scoring just four points on 2-for-7 shooting (0-for-3 from three-point range) in his 20 minutes on the floor. Marcus Smart, though he was on the floor for Boston’s fourth-quarter comeback, was far too interested in trying to coax the officials into calling offensive fouls (and argue when they didn’t blow the whistle) than get back on defense. Crowder, who was Boston’s leading scorer with 16 points on 4-for-10 shooting, played well in spurts, but looked disjointed for long stretches.

And while they managed to get to the free throw line 37 times, the Celtics missed 12 freebies.

Head coach Brad Stevens has had no problem calling out his team following disheartening showings this season, but was quick to take the blame for their slow start on Wednesday.

“For the first two and a half quarters, I didn’t think we played, and that’s the frustrating part,” Stevens said. “I told the guys, that’s got to be on the coach if that team’s not good on that night, first and foremost.”

As poorly as they played over the first 36 minutes of the game, the Celtics managed to find a spark in the fourth quarter thanks to an interesting lineup. Stevens employed Crowder, Phil Pressey, Marcus Smart, Jonas Jerebko and Luigi Datome to cut into Miami’s 20-point lead, and they did just that.

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An 18-6 run had the Celtics within eight points with just under five minutes to play, capped off by a Gigi three-pointer. A few minutes later, Stevens drew up a beautiful play to get Crowder open under the basket out of a timeout, but the forward missed the open layup that would have cut Miami’s lead to five.

That all but sealed the Celtics’ fate. Crowder promised to be better following the game:

The Celtics outscored Miami 24-11 in the game’s final frame, but it was too little too late.

“In the fourth quarter they played with great emotion and hard as heck and got us back into it, but then we just didn’t make very good decisions or play with poise late, which sometimes happens when you’re trying to mount a furious comeback,” said Stevens.

It never should have come to that for Boston. But they found themselves playing catch up due to their poor start, and didn’t have enough to overcome such a big deficit in the end.

The Celtics, now 31-40 for the season, remain in the eighth and final playoff spot in the Eastern Conference. The Indiana Pacers were victorious over the Washington Wizards on Wednesday and also own a 31-40 record, with Boston owning the tie-breaker. The Charlotte Hornets lost to the Nets, dropping them half a game behind the Celtics and Pacers — and tied with Brooklyn — on the outside looking in.

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But the most harmful part of Boston’s loss on Wednesday was not picking up ground on the Heat. A win would have put Miami and Boston into a tie for the seventh spot, but the C’s now find themselves two games back of Miami with 11 games to go. That seventh seed could now be out of reach, and the Pacers are (no pun intended) keeping pace for the eighth spot.

The Celtics have been very good at bouncing back this season, but Wednesday night was one big missed opportunity for the team. With less than a dozen games remaining, they won’t be able to have too many off nights the rest of the way and expect to make the playoffs.

Hopefully their vow to be better carries over for the next three weeks, and beyond.

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“We just have to know what’s at stake, as a unit,” said Crowder. “Each game right now, we’re playing for something. If that doesn’t motivate you to come out and be prepared and be ready to go, I don’t know what will.”