ESPN's Adrian Wojnarowski reported it first, noting the injury was being called a sprain but was still under scrutiny.
With Marcus Smart's return this season hanging in the balance, he and the Celtics will now turn to a second opinion for answers. The guard suffered the injury while trying to retrieve a steal during Sunday night's loss to the Indiana Pacers.
The Celtics hobbled through with a 6-5 record during that stretch, testament to how much they will miss Smart again, and with only 15 games left in the regular season. Despite making a season-high-tying six of his 12 3-point attempts on Sunday, Smart is shooting just 30.1 percent from long distance this season and 36.7 percent overall.More news: China steel cuts too slow for West
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"Sore, but it will be all right", Smart told reporters after the game.
Smart, 24, is averaging 10.2 points, 4.8 assists and 3.5 rebounds this season.
The Boston Celtics are dealing with a lot of injuries, some more serious than others.
Though he proceeded to play impressive basketball the rest of the game, the Celtics now fear he could need surgery that would keep him off the court for the rest of the season.