An injured Marcus Smart, now a foe, is still on the minds of the Celtics before Sunday’s game in Memphis

Celtics fans won't get to see Marcus Smart in his new Grizzlies threads Sunday after the guard injured his ankle and will miss the game in Memphis.Ryan Sun/Associated Press

MEMPHIS — Marcus Smart used to think about spending his entire career with the Celtics and possibly having his number join the others hanging from the TD Garden rafters.

Boston certainly did not enter this offseason intending to trade its longest-tenured current player, but opportunities arise suddenly in the NBA and team decision-makers must always be prepared to strike.

In June, when there was a chance to acquire the 7-foot-3-inch Kristaps Porzingis, Celtics president of basketball operations Brad Stevens made the tough decision to part with Smart, sending the 2022 Defensive Player of the Year to the Grizzlies in a three-team deal.

Smart was blindsided by the move but not angry about it. He understood it was just part of the business and later in the summer still welcomed a large Celtics contingent to his wedding, including co-owner Wyc Grousbeck.

Smart was scheduled to face his former team for the first time Sunday night, when the Celtics visit the Grizzlies. But the ankle injury he suffered last week is expected to sideline him for 3-5 weeks, so he will just be a spectator.

Still, it will offer him a brief chance to reconnect with his friends within the organization. The Celtics have roared to the top of the NBA with a 10-2 start, and forward Jaylen Brown says it’s important to remember that Smart was instrumental in laying the foundation for the franchise’s swift rebuild.

“A lot of what we’ve built here, being able to win, being able to get to multiple conference finals, being able to establish an identity in basketball, a lot of that is Marcus,” Brown said. “Him diving on the floor for loose balls, hustling, and being a game-changer is what he kind of does and how he’s made a name for himself. He’s a large contributor to why we’ve had success. He’s no longer with us, but we’ve got to give credit where credit is due.”

Smart is averaging 12.5 points, 5.0 assists, 2.5 rebounds and 2.0 steals per game for Memphis. But it has been a frustrating start for the 2-9 Grizzlies, who have been ravaged by injuries and are still awaiting the return of star guard Ja Morant following his 25-game suspension.

Even though Smart will not have a chance to take the court against his former teammates, he should be back for the Feb. 4 game at TD Garden.

“The relationship is still there,” Brown said. “Just because now we’re on different teams in different jerseys, that’s still a brother of mine. So it’ll be good to see him down in Memphis. I know he’s out with an injury for a little bit, but if he pokes his head out, it’ll be good to share some time with him.”

Brown is questionable Sunday with a right adductor strain. He suffered the injury in the final seconds of Boston’s win over the Raptors and was critical of the new court used for the in-season tournament, saying it was slippery throughout the game.

Picking up the slack for Tatum

Even when Jayson Tatum is struggling with his shot, the Celtics have guys like Derrick White who can step up offensively.Barry Chin/Globe Staff

All-Star Jayson Tatum struggled mightily from the 3-point line in the 108-105 win over the Raptors on Friday night, making just 1 of 11 attempts. It was the first regular-season game of his career in which he failed to make more than one 3-pointer when shooting at least 10.

The Celtics were certainly not concerned afterward, though. If anything, they viewed it as an encouraging sign that they’d found a way to win during their superstar’s off night.

“For his standards, it’s obviously a tough game and something that happens very rarely,” guard Derrick White said. “We just have a bunch of guys that’ll step up and make shots. Sam [Hauser] had a run in the first half that was huge for us. Al [Horford] hit the back-to-back threes in the second half that were big for us. So, just having guys that’ll pick you up when 90 percent of the time he’s going to be who he is and when he’s not, we have other guys who will step up.”

White chipped in, too, drilling the game-winning 3-pointer with 26.8 seconds left.

Coach Joe Mazzulla pointed out that the vast majority of Tatum’s 3-pointers were wide open and well within the flow of the offense. He was not forcing the issue.

“And it says a lot about Jayson and Jaylen,” Mazzulla said. “It says a lot about our team. And you have Jrue [Holiday], [Porzingis] and D-White bring us home. So, that’s going to happen. We’re going to need all those guys.”

Undefeated in group play

The Celtics are 2-0 in group play in the inaugural in-season tournament. They are followed in the Eastern Conference’s Group C by the Nets (2-1), Magic (1-1), Raptors (0-1), and Bulls (0-2).

The Celtics defeated the Nets in their tournament opener Nov. 10. They will visit the Magic Friday before concluding group play at home against the Bulls on Nov. 28. The winners of the six groups as well as one wild-card team from each conference will advance to the quarterfinal round. The semifinals and final will be played in Las Vegas on Dec. 7 and 9.