He (jack) might not be exactly vital just yet, but remember he's only 21 and has been bugged with injuries which have deprived him of action for most of his short professional career. I have no doubt that if he can overcome injuries and stay fit, he'll be the lynch pin of our team in very short order.
As for Rosicky, a player whom I've always highly rated, no one is questioning his directness in play (something which Jack is widely known for as well), but I'm sure there have been games where he also looked completely out of sorts and wasn't exactly causing things to tick in our midfield. Therefore, I won't place too much expectations on him till he can put together a consistent run of being a bit of a maestro in the middle of the park.