Smart underwent an MRI earlier Wednesday, which revealed a "partial avulsion of his left oblique abdominal muscle off of his iliac crest." The team said he is expected to resume basketball activities in four to six weeks.
The Celtics' timeline for Smart's return implies the guard could miss the team's first two postseason series. Boston plays the Indiana Pacers in the first round, with a likely matchup against the top-seeded Milwaukee Bucks in the second round.
Smart's injury occurred at the 6:44 mark in the third quarter of the Celtics' 116-108 loss to the Orlando Magic on Sunday. The 25-year-old guard has averaged 8.9 points, 4.0 assists and 2.9 rebounds, and his defensive presence has provided a significant boost to the team.
Jaylen Brown will likely slot back into the starting lineup at shooting guard in Smart's absence. The Celtics struggled at the beginning of the season, leading to Smart replacing him in the starting five.
Smart signed a four-year, $52 million contract as a restricted free agent last off-season. He is shooting a career-best 42.2 percent from the field, including a career-high 36.4 percent from 3-point range, this season.