Anfernee Simons is a productive NBA player because he's a high-level scorer. The 26 year-old is an elite 3-point shooter, can create offense for himself off the dribble, can find his shot in the midrange, and has always been better than average finishing at the rim. Those are all important traits for a guard.
But Simons has never been a multi-faceted player, and Celtics fans are finding that out quickly about the guard on a $27 million expiring deal. When his shot is working, he's a firestarter. When the shot isn't falling, Simons doesn't produce in enough other ways to be a consistent part of a rotation.