Earlier Wednesday, Celtics coach Brad Stevens called Smart's actions "unacceptable."
After missing a 3-pointer midway through the fourth quarter, Smart appeared to gesture toward a fan courtside at TD Garden. Soon after, as he retreated on defense, Smart seemed to motion for the fan to sit down.
The top-seeded Celtics displayed obvious signs of frustrations while enduring a 111-97 loss that put them in an 0-2 hole as the first-round series shifts to Chicago for the next two games.
"Obviously, I've been told of the gesture," Stevens said Wednesday during a conference call. "I saw a brief video clip of it but, from my clip, from the angle that I saw, I couldn't tell. But obviously if he made a gesture, it's unacceptable, and that will be handled by the league."
Smart finished with 13 points on 6-of-11 shooting over 27 minutes. The Bulls were up 14 with 6:11 remaining when Smart missed the 3-pointer before the incident.