You can get inferior personnel to perform better than it would seem they should. But to think an HC can take a bunch of scrubs and make them a very good team isn't realistic.
I don't know if McVay is the Rams best HC ever. But he's the only one to take them to 2 Superbowls, winning one of them. And BTW: in that Superbowl win he was down TE1 and TE2 and then TE3, WR1b in Woods, and lost his replacement before halftime (OBJ), and down both starting Safeties. McVay coached his butt off.
