X-rays on Smart's ankle were negative, a source told ESPN's Jeff Goodman. The team said Smart would have an MRI later Friday night.
Smart appeared to step awkwardly on the foot of Indiana's Lavoy Allen and immediately grabbed his left ankle after crashing to the floor. Team doctors attended to him on the court before a stretcher was summoned. His leg was immobilized and he then was taken to the locker room, giving a thumbs up as he was wheeled off.
"It was really scary and then, obviously, he was writhing in pain, so that makes it even more scary," coach Brad Stevens said.
"We're hoping it's a sprain, but we don't know that. So obviously a great deal of concern for him, No. 1. And hopefully he can get back sooner rather than later because he's really playing great basketball, and I'm sure it's really disappointing to be out five games into your rookie year, even if just for a little bit of time. That's what we hope it is."
Stevens did say he liked the way his team rushed to check on its fallen teammate.
"I'm glad to be part of [this team]," the coach said. "I'm glad to be a part of a team that feels like that."
A chant of "Marcus Smart" filled the Garden as Boston players stood solemnly around Smart.
"It was very difficult; you hate to see a teammate go down, especially when he's playing well," forward Jeff Green said. "Marcus is a soldier, though. He's going to get himself back together quick. I know he's going to work hard to get back. Tough loss for us."
Smart was the No. 6 overall pick in June's draft. He's carved out an immediate role with the Celtics as a top guard option off the bench.
Smart was averaging 7.8 points, 2.8 rebounds, 1.5 assists, and 1.5 steals over 21.5 minutes per contest in Boston's first four games of the season.
"I'm going to reach out to him like always and be a good teammate, but that's not just because I got to be a good teammate, that's because that's family," forward Jared Sullinger said.