Of course Marchisio is ready to start. Easy choice really, there's no way Allegri is benching him in this situation...
He isn't gonna be 100% after just a week's worth of training and with no competitive match time after returning from injury.
If he plays, he won't be good either, but it'll show if he can play vs City, and starting vs Chievo would be the best preparation for City. I'd field a Marchisio playing at 80% of his capacity vs City. We'll need him in that game if he's good/fit enough to start.
Given the upcoming City game, starting him vs Chievo isn't a bad idea, but throwing him in a competitive game like this from the start when he isn't fully match-fit definitely increases the chance of him getting re-injured.
There's no good or bad decision here.