In most leagues Marcus Smart was probably either drafted later or not at all since he isn't expected to start over Rajon Rondo. There may have been a bump in his value when Rondo broke his hand and there's going to be a portion of fantasy basketball players that believe that Rondo will be traded, leaving Marcus with the starting job. I don't happen to share that belief.
I think Smart will get immediate exposure to playing time due to his ready-for-the-NBA defense. So far he's averaging 19.5 minutes per game through 3 games. He's going to get steals (2.0 per game so far) so he might be worth keeping around just to fill up that stat. But he's only going to put up modest scoring figures until he improves his jumper (which he's being encouraged to shoot, regardless of shooting percentage) and thus far his assists aren't going to make anyone forget about Rajon (1.5 per game).
Still, he does have some great upside and he will continue to get regular rotation minutes as the Celtics feel their way through another rebuilding season. Recommendation, if you need steals or a backup point guard that will get minutes, consider Smart for your fantasy team. Also, if you are playing in a FanDuel league, he might be a nice, cheap play on any given night when you need a point guard option.