I keep hearing about half of our fan base say he's coming back, yet I don't recall but one person state a reason.
I don't have any inside knowledge either way, but a few reasons he may stay include:
1) The way he consistently drops his head and bulls his way to the hoop works great ~90% of the time in college; it hasn't worked against teams with talented big men. Fortunately, most college teams do not have truly talented big men; but most NBA teams do. For that reason, I think, right now, he is much better suited to the college game, where he is a better athlete than most of the guys he goes up against. He won't be the better athlete against most NBA defenders he will face, and he has many areas of his game upon which he can still improve to round out his skill set.
2) Like Amile, Grayson has consistently been a scholar athlete (consistently all-academic ACC). Unlike some guys who may see Duke primarily as a stepping stone to the NBA. Grayson appears to be here for the academics as well as for basketball.
3) Watching him, I get the sense that he loves playing for Coach K and being part of the Duke team; and he has to love the idea of making another title run next year. While next year's team will be very talented, there is no way that he doesn't start and get as many minutes as he can handle as long as he's healthy.
4) The draft projections I have seen have him slotted in the 25-30 range. That doesn't offer much room for error in terms of potentially dropping to the second round and/or possibly never seeing major minutes in the NBA.
For those reasons, I am optimistic that he will stay; but I have no idea whether he might feel pressure to "grab the money" now while it's there. I would love to see him back next year.
