Marcus is still in search of just where his best opportunities are on offense in the NBA. He's having a difficult time finding his path.
In college his physicality gave him an advantage in his scoring on offense. I was easy for him to bull his way into the paint and get to the rim, and score. In the NBA everyone is bigger, longer, stronger, and more talented. He now has to settle for getting into the paint, and dumping the ball to the open shooter man, or taking long 3's.
Unfortunately this will not lead to points scored, not unless his outside shooting improves dramatically.
Rozier, and his game is based more on speed, quickness, ball handling, he looks to be able to get to the rim quite easy, due to his athletic ability and speed, his defense will be strong, as he learns the NBA game.
By next year Rozier will force the Celtics to make a decision on who will be the point guard of the future.