Boston Celtics guard Marcus Smart was forced to leave Sunday's game against the New York Knicks after taking an elbow to the oblique area from Kevin Knox in the third quarter.

Smart was then ruled out for the rest of the game, according to A. Sherrod Blakely of NBC Sports Boston.

Smart got into good position after rotating back to defend Knox, but the second-year forward  got into his body as he attempted to get into the lane.  As can be seen from the video, Smart almost immediately grabbed his midsection and doubles over in pain as he goes to ground.

The Celtics are awaiting further word with respect to the injury, but this has to be concerning. Smart suffered a torn oblique at the end of last season that kept him out through the first round of the playoffs, as well as the first three games of Boston's Eastern Conference semifinal against the Milwaukee Bucks.

Smart was expected to miss four to six weeks with that initial oblique tear, but he resumed basketball activities well ahead of schedule. As positive as it was to see Smart looking healthy, early returns always bring about questions in terms of long-term health.

It would be a tough break for the Celtics if Smart is out for a similar time frame. He is one of the best defenders in the rotation and has been just as valuable off the bench as he is as a starter. Not to mention, he is invaluable as a guy that can guard bigs, especially given Boston's lack of frontcourt depth.