It appears that he's there primarily as McVay's yes man and possibly the errand boy and apprentice.

I think McVay is breaking him in gradually, like he does his players. When he sees Coenas thoroughly indoctrinated, completely trusted to take over the system and run it as McVay does, he might be given a freer hand.

He wasn't brought in to run the offense. Maybe he runs the O in practice, reports and confers with McVay; we all now his title, but does anybody know exactly what his specific tasks are?