The reason I say that is because Castle seems to have been banging on about playing PG since before he was drafted. He played a big part in helping turn the Spurs around last season.
I agree that you're probably right that it's not realistic, but in this case, I can see it being a possibility. Maybe he asks for a trade if they didn't agree to play him at PG?
Then couple that with the fact that Chris Paul likely had an agreement to start all season last season, it seems the Spurs are starting to bend. I never would have expected them to do something like that ever, yet it seems like they promised Chris Paul a starting spot all of last season. It was always you gotta earn your spot and nothing is promised, but it seems to me they have made some promises as recently as last season with CP3. If Castle is really adamant of playing PG, maybe they have tried appeasing him. This would have also happened during the off-season when they had no idea if they'd be a good team or not, so maybe the caved and went along with it. Why has Mitch not taken him off the ball with his recent struggles the past month or so when you have a PG you just gave a max contract to?