Smart originally suffered the injury against the Magic on Sunday.
Celtics guard Marcus Smart could miss the first two rounds of the NBA playoffs due to a torn oblique injury, according to ESPN's Adrian Wojnarowski.
Smart originally sustained the injury early on during Boston's loss to the Magic on Sunday. He was ruled out in the regular-season finale against the Wizards because of the ailment.
Smart appeared in 80 games (60 starts) for Boston this season. The 24-year-old averaged 8.9 points, 4.0 assists, 2.9 rebounds and 1.8 steals.
The Celtics are locked in as the Eastern Conference's fourth seed and will face the Pacers in the first round.