Coach Brad Stevens noted, per Celtics announcer Sean Grande, that Smart underwent a scan on his thumb. Although the thumb "didn't look too bad," the team has opted to take the safe route.
Smart is just one of several Boston players who will miss Monday's game. As a result, the Celtics' depth will be put to the test on Monday, as Tim Bontemps of ESPN pointed out:
This is another setback after he suffered a torn oblique leading into the 2019 playoffs and appeared in just 54 games in 2017-18.
Smart averaged 12.9 points, 4.9 assists, 3.8 rebounds and 1.7 steals a night in 60 games in 2019-20 and has followed up with 12.6 points and 6.7 assists through his first seven appearances this season.
The Oklahoma State product does a little bit of everything for the Celtics, but his defense stands out the most. He routinely defends the opponent's best wing and backcourt players and held them to 2.2 percent worse shooting from three-point range and 2.5 percent worse shooting overall than their normal averages last season, per NBA.com.
That defense is one reason the Celtics have championship expectations—when the roster is healthy.