By Matthew Geagan, CBS Boston

BOSTON (CBS) — Marcus Smart is usually a pretty wise player, someone who knows what’s going on in any given situation. But on Monday night, the Celtics guard suffered the ultimate brain cramp that helped spark a Pelicans run in yet another Boston loss.

The mental hiccup occurred with 4:31 left in the third quarter and the Celtics down by four points. Pels forward Brandon Ingram had the ball knocked away, and he ended up on the floor along with Celtics guards Evan Fournier and Payton Pritchard fighting for a loose ball, leading to a jump ball. The Pelicans had just 0.3 seconds left on the shot clock, so Ingram wasn’t even going to attempt to jump against Pritchard.

The Celtics won the tip, and the ball ended up in Marcus Smart’s hands. That 0.3 seconds only applied to the Pelicans, but that is something that Smart was not aware of, or forgot, or just… well who the heck knows what Smart was thinking. Never afraid to put up a shot at the end of the buzzer, Smart heaved up a prayer that he didn’t have to heave up, costing the Celtics a pretty important possession.

It’s something that I’ve never seen as a basketball fan, and quite frankly, I’m not sure I’ll ever recover from seeing. Smart is usually one of Boston’s — for lack of a better word — smarter players on the floor for Boston, but even his head is out of the game at this point. He was spotted arguing with teammate Kemba Walker after the heave, apparently thinking he was in the right.

It was a costly mental mishap for Smart and the Celtics. Boston gave up an offensive rebound that led to an Ingram three on New Orleans’ gift possession, and Smart heaved up another brick on the other end of the floor. The Celtics became unraveled after Marcus’ heave, with the Pelicans rattling off a 20-8 run to balloon their lead to 16 early in the fourth quarter.

The Celtics went on a late run to make things interesting, cutting it to a three-point game in the final minute, but it was too little too late — the story of the 2021 Boston Celtics. Smart was in the thick of that run, but when Boston’s deficit was back up to six with 24.7 seconds left, he had had enough and got himself ejected for arguing over an uncalled charge that he believed he forced at the other end.

The fans that were inside TD Garden for the first time in over a year got the full Marcus Smart Effect on Monday night, but unfortunately, it wasn’t the good version. On good nights, Smart’s flub would be something the team could laugh off. Something they would shrug off and keep in the back of their minds for some friendly ribbing after the game. But Monday was not a good night for the Celtics, and they let everything spiral out of control.

It’s a sign that the team just doesn’t have the mental makeup to overcome such oddities this season, something we’ve been hoping would be proven wrong on several occasions. That wait continues, and much like Monday night’s game, it will probably end up being a too little, too late situation.