By Matthew Geagan, CBS Boston

BOSTON (CBS) — The disturbing trend of hopelessness continued for the Boston Celtics on Sunday, as the team blew a 24-point lead in the second half to the New Orleans Pelicans. Whenever the Celtics take a perceived step forward, they quickly tumble back in disappointing fashion, and are now stuck in a malaise of mediocrity.

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If only the team were mediocre on Sunday, maybe Boston would have been able to hang on to that large advantage and earn its second straight win. Instead, the Celtics let New Orleans storm back for its biggest comeback win in franchise history. It’s another embarrassing Boston defeat to add to the giant pile of embarrassing defeats, dropping the Celtics back to .500 at 15-15 on the season.

At this point, it seems like these Celtics could find a way to fall out of a basement window. Just when you think that Boston has found its Sunday’s best against the Pelicans, they reverted back to their normal form: A team that becomes an absolute puddle in the fourth quarter. The Celtics let their 24-point lead evaporate by going 6-for-22 in the fourth quarter, while the Pelicans feasted in the paint. Zion Williamson scored 24 of his 28 points in the second half, dominating under the basket against anyone the Celtics threw at him.

The Celtics had no answer while the game crumbled around them, outscored 34-21 in the fourth and 12-7 in overtime. Boston is now 7-12 in games separated by five points or less in the final five minutes, and tout a minus-33.2 rating in those crunch-time situations. The Celtics are the 29th ranked team in the NBA, ahead of only the Cleveland Cavaliers, when it comes to fourth quarter rating, sitting at a putrid minus-7.8.

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Kemba Walker hit just one of his six shots — four of which came from downtown — during the fourth quarter. He added another three misses, all from deep, in overtime. Jayson Tatum was an uneven 3-for-9 over the final 17 minutes, and Jaylen Brown had just seven points off 3-for-6 shooting in the fourth and overtime. The Celtics made no real attempt to attack the basket and get to the line as their lead slipped away, attempting just five free throws in the fourth quarter. Two of those were early in the frame off a pair of technical fouls by JJ Redick.

The Celtics keep pushing the narrative that they’ll learn from these meltdowns and eventually put it all together. But whereas Brad Stevens teams used to thrive in the face of adversity, this iteration crumbles and folds. Danny Ainge spoke of maturity issues on the roster last week, and so far this season, the Celtics have shown zero signs of growth in the areas that have haunted them the most.

We’re just a month away from the NBA trade deadline. The massive TPE that Ainge received from the Hornets for Gordon Hayward’s departure over the summer, once seen as a way to add to what should have been a contending team, is now the only real hope of adding an impact player or two to the roster. Ainge has to hit on any move he makes over the next five weeks if Boston wants any shot at sitting at the adult’s table come the postseason.

But even that exception is no guarantee to right the ship. That will ultimately be up to the players and the head coach that are already in place. So far this season, the Celtics have shown that they’re the pinnacle of mediocrity, a 15-15 team that can roll out an impressive win one night, and lose in historic fashion the next.

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Until the pieces in place show that they can are ready to learn from that heap of disappointing losses, the Celtics are going to be stuck in this vat of mediocrity.