The OC isn't an empty position, even if McVay calls plays. You want someone who is capable of running the offense in practice and meetings. You want someone who can help with play design, gameplanning, and scripting plays. You want someone who can amplify your weak points as a coach. And most of all, you want someone who can take over the playcalling if it gets to be too much to do it with your HC duties.
I wouldn't trust Boras to coordinate colors, much less this offense. McVay needs someone he can trust to possibly handle this offense in the future.