On Thursday night, the Celtics sent another warning to the NBA with one of their best nights of a season. Jayson Tatum was named an All-Star starter again as the C’s produced a record-breaking offensive performance in a rout of the Heat. The vibes were high as they left Miami to come home for a long homestand.

Two nights later, Tatum had to resort to sitting the entire fourth quarter at the end of the bench with a towel draped over his back. It was not for a good reason.

The Celtics followed up arguably their best performance of the season with their worst. For the first time this season, the C’s looked completely inept. They looked lost in a brutal 115-96 loss to the Clippers at TD Garden that wasn’t even that close.

Jayson Tatum scored 21 points to lead the Celtics, who put up enough bricks to build a few new buildings in downtown Boston. After lighting up the nets in Miami, they shot 35.6 percent from the field and went 10-for-40 from three. They missed countless layups. Joe Mazzulla couldn’t use up his timeouts quickly enough. They were booed on their home court for the first time this season.

Kawhi Leonard scored 26 points, Paul George had 17 points and old friend Daniel Theis had 18 points off the bench – more than any Celtic not named Tatum – for the Clippers, who were on the second leg of a back-to-back. You’d be forgiven if you thought it was the Celtics who were on the back-to-back.

The C’s didn’t have Kristaps Porzingis, but that was little excuse. No one really showed up. Derrick White and Al Horford combined to go 0-for-13. Jaylen Brown had eight points on 3-for-13 shooting. Jrue Holiday had seven points on 2-for-11 shooting. The Celtics, who started 20-0 at home, have dropped two straight games at the Garden to potential NBA Finals opponents after their loss to the Nuggets last week. Both losses were defined by offensive struggles.

Saturday was far worse. They scored 39 points and trailed by 16 at halftime after a first half that didn’t seem like it could get worse. But then it did. Tatum scored seven consecutive Celtics points over the first two minutes of the third quarter and they looked like they possibly had a comeback in them.

They did not. The offensive struggles that ensued were shocking, as the Celtics went on a scoring drought of six minutes, 34 seconds. After the Celtics cut their deficit to 12, the Clippers went on a 21-0 run. In that span, Mazzulla called two timeouts. Nothing could help them. Bricks piled up. Turnovers led to easy baskets. Holiday had one turnover in which he passed out of bounds to nobody. Tatum, after a couple of misses, was called for a technical foul complaining for a foul.

It all unraveled as the Celtics went down by as many as 36 and never recovered. They scored just 60 points through three quarters. Mazzulla pulled his starters at the start of the fourth quarter.

The Celtics shot 26.9 percent in the first quarter, including 2-for-12 from deep, but only trailed by five. It seemed like a minor win. Surely the Celtics’ shots would eventually start falling.

They never did.

Sam Hauser made a 3-pointer to begin the second quarter to put the Celtics within three. It proved to just be a glimmer of hope. Even without their top two starts in George and Leonard, who sat to begin the second, the Clippers dominated. Harden drew a flagrant foul on Jaylen Brown while shooting a 3-pointer. The Clippers were putting in the extra efforts as they took a 47-32 lead.

The Celtics created a spark by turning defense into offense to cut the deficit to 47-37, but it didn’t last. The bricks continued to clang. They scored 39 first-half points, their second lowest of the season. It was just one more than from their blowout loss to Milwaukee a few weeks ago, when they were on the second leg of a back-to-back and in the midst of a grueling schedule. They had no such excuses on Saturday night.