I'm not sure if there is a squad size limit in the championship, but even if there is, Petrov has to take a place. Even if he only a makes 1 appearance it will still be worth it. He will be such a big influence on everyone, but especially the likes of Grealish and Gardner.
I was one of those that didn't think he was that a player great pre illness (even though looking at the team in his absence has certainly shown that he was way above everyone else we've had recently). But even I can see that his attitude and behavior will be hugely influential on the team, as a registered player it will have more impact than that of a coach (which I think he'll take up after 1 yr).