Celtics guard Kemba Walker briefly left Boston’s game against Charlotte on Sunday with pain in his left side. He returned and said he couldn’t explain what happened. Curtis Carlson/Associated Press

There was some confusion and uncertainty but most of all groans midway through the second quarter Sunday as the Celtics took on the Hornets. It wasn’t because Charlotte was hitting droves of 3-pointers, though that wasn’t helping.

No, it was because Kemba Walker seemingly hurt his side on a non-contact play. Walker sprinted on the close-out on a Terry Rozier 3-point attempt, but by the time he got there, Walker wasn’t able to rise to contest the shot. Walker grabbed at his left side, said a few words to the bench then immediately jogged to the locker room.

Even Walker came away from the experience stumped.

“I can’t even really explain it, to be honest,” Walker said after the Celtics’ 125-104 loss to the Hornets on Sunday. “I don’t know. I just felt some weird pain on my side. I can’t really explain it. It was just pretty painful at the time.”

Walker ended up coming back to the game, to the relief of Boston fans. Right when the Celtics were getting healthy, another setback to Walker would’ve been a brutal blow.

While there were moments where Walker grimaced in pain, he stayed in the game. He finished with 20 points, six rebounds and four assists in 29 minutes.

Celtics Coach Brad Stevens said he had no clue what happened on the play. All Walker told him before retreating to the locker room was “get me out.”

“I’m fine right now,” Walker said after the game. “Yeah, I’m still hurting a little bit, to be honest. But I guess I’ll get better information (Monday), I guess, about how I feel when I wake up.”

THE ONLY PLAYER on the injury report Sunday was Robert Williams III, who continues to deal with left knee soreness. Williams missed his sixth straight game and there still doesn’t appear to be a timetable for his return.

Stevens said last week that Williams was scheduled to get some workouts in ahead of getting back on the court. But he wasn’t able to clear the workouts, which made any sort of development murky.

“I think he’s getting better, so we hope that again, he’s back at some point soon,” Stevens said pregame. “But just not quite ready (Sunday).”

JAYLEN BROWN returned from a left shoulder injury, though it was a slow start to the game for the All-Star. Brown tied with Walker for the team-lead with 20 points, but it was inefficient as he shot 8 for 22, though he added eight rebounds.

Evan Fournier had another game to forget, shooting 0 for 3 and finishing with zero points in 20 minutes. It was the second straight game where he struggled since returning from contracting COVID-19, but he’s still getting his legs under him.

Fournier said he probably needed at least a few more days from a conditioning perspective before his return, but he accelerated that process because he said he needed more reps with his new teammates.

THE CELTICS  looked like they were about to make it a game late in the third quarter. The deficit was down to nine points and Marcus Smart was bringing the ball up. He seemed like he drew a foul in the act of shooting, which would have earned him three free throws.

Then the worst-case scenario happened. There was a no-call on the foul as the ball went toward half court. Miles Bridges heaved a 29-foot, double-clutch 3-pointer as time expired, knocking it down. It ended up as a six-point swing and boosted the Hornets’ lead to 12 points.

The Celtics couldn’t muster the energy to pick up the pieces of the comeback attempt from there.

THE CELTICS have lost two of their last three games, a brutal time when all the teams around them in the standings keep winning. Boston is now tied with Miami for sixth place at 32-29. A few more losses and the Celtics could find themselves back in the play-in category.

Boston is only two games behind the fourth-place Knicks, but New York has been on a tear winning nine straight games. It’ll be difficult, too, catching the fifth-place Hawks as Atlanta has the tiebreaker over Boston.

The Celticss have a rematch against the Hornets on Wednesday, which will be another important game in the playoff race. But first, a Tuesday game against the Thunder, who are on a 13-game losing streak.