I don't believe Castle will start for the Spurs if they draft him -- even if he's taken fourth overall. At least he wouldn't start a PG. That would be like starting Sochan at PG all over again. I know a lot of people hate Jones, but he's ... fine. He's solidly above-average in most aspects of being a point-guard. He even shot 39 percent from three as a starter. He has one of the best defensive grades on the team. It's not likely that any PG the Spurs draft would come into the season as a starter. Castle coming off the bench and getting some ball-handler minutes in the second unit seems like the best way to accommodate his desire to play the position, but it should be noted that him wanting to play PG doesn't obligate the Spurs or any other team to play him there. He'll play where he fits best and where his development takes him.
That might be small-forward. Tonight noted that Castle would lose his size advantage if he moved positions, but I think that's an antiquated way to thinking. The Spurs are not going to play straight up against teams with positions direct-guarding their counter parts. Doing so would be a disservice to the generational defender anchoring the paint. The Spurs are absolutely going to be sending guys like Sochan and Castle after the best scorers and rely on their ability to defend multiple positions to give them a ton of flexibility. Castle would bring a level of "stickiness" that would provide the Spurs with better means to deny the ball to certain guys. It would be wasteful to use that ability primarily on PGs, who will likely have the ball to start the possessions.