The problem with Dion is not his minutes, it's how he was used.
Under RW, He was used offensively, and excelled. He put up points, hit like a Mac Truck, and was actually better defensively while having his best best possession numbers.
RC tries to use him as a pure Defensive D, which is not his forte. He ends up trying to do too much, and making more mistakes as a result.
Give him better support (like a true shut down pair), let him go offensively and you'll see his defensive game improve dramatically.
He's a legitamite top pairing defensemen - but like all players he needs to be put in a position to succeed.