Smart was coming off a screen set by Al Horford on the wing at Scotiabank Arena late in the second quarter when he stepped awkwardly on his right ankle and completely rolled it on its side.
Smart immediately reached for his ankle in pain before he fell to the court, committing a turnover.
Smart was down for some time before the Celtics’ medical staff helped him up and walked him off the court slowly. The team then ruled him out a short time later.
Smart finished the day with 2 points and 4 assists in 16 minutes.
The Celtics, despite scoring just 5 points in the final 5:53 of the game, held on to beat the Raptors. Jaylen Brown led the way with 27 points and 8 rebounds for Boston, and Grant Williams added 25 points off the bench.
The Celtics entered Saturday’s game in Toronto on an eight-game win streak. They beat the Golden State Warriors in overtime on Thursday night, where Smart had 18 points and 5 rebounds in 38 minutes.
Pascal Siakam led the Raptors with a near-triple-double with 29 points, 10 assists and 9 rebounds. Gary Trent finished with 22 points.
Celtics coach Joe Mazzulla said after the game that Smart will be considered “day-to-day” moving forward. X-Rays on his ankle came back negative, though it’s unclear how much time he will miss, if any at all. The Celtics will take on the Orlando Magic next on Monday before traveling to Miami for a game against the Heat on Tuesday.