Celtics coach Brad Stevens said Smart suffered a "subluxation of the proximal tib-fib joint." Smart limped off after banging knees with Brooklyn's Thomas Robinson early in the fourth quarter. The team initially classified it a bone bruise, but further testing revealed the dislocation.
"They say a couple weeks-plus," Stevens said before Sunday's rematch in Brooklyn. "And there's no real end beyond a couple of weeks. It could be a couple weeks, it could be longer than that. Hopefully it gets a lot better sooner because obviously he's a big loss for us."
Smart will not require surgery, but Stevens said the team will reevaluate him as he starts the rehab process.
"He'll listen to our doctors and they'll work together," Stevens said. "They've gone and taken a bunch of images of it and they wanted to make sure. They've actually asked for second opinions on it, because it's a really unique injury. But it's not a long-term thing."
How will the Celtics otherwise fill Smart's minutes?
"I think Jonas [Jerebko] will play some of those minutes; I think one of our young guys will play some of those minutes, to be determined on the night; and we'll go from there," Stevens said. "That's why we have guys that are capable of stepping in and helping."
Smart missed three games earlier this month due to a sprained big toe. Smart appeared in 67 games last season, but he missed extended time early in his rookie season due to an ankle injury.
Smart is averaging 9.8 points, 3.8 rebounds, 3.2 assists and 1.8 steals over 27.2 minutes per game. His defensive tenacity is a big reason Boston owned the best defensive rating in the league entering the weekend.