Per an official statement from the Celtics, Smart is expected to miss between four and six weeks as an afternoon MRI revealed a "partial avulsion of his left oblique abdominal muscle off of his iliac crest".
His absence is a severe blow to Boston, given his ability to guard most positions on the floor.
The timeframe likely removes Smart from the first two rounds of the National Basketball Association playoffs.
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Smart suffered the injury Sunday in a loss to the Orlando Magic. 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. Presuming Brown does, in fact, return to the starting lineup now, Boston will be hoping he can carry that momentum over into the postseason - though Brown has been dealing with back issues of his own over the past week or so.
Smart, who signed a four-year, $52 million deal with Boston as a restricted free agent last summer, shot career-bests from both the field (42.2 percent) and from deep (36.4 percent) this season.
Smart is a key part of Boston's gameplan. "We've thought ever since he got here that he's one of the best defenders in the league".