Brian Kelly's hybrid pro-style / spread offense is all centered around a quarterback's ability to throw a highly accurate short ball, as well as a decently accurate medium range ball. Without this, the defense doesn't have to respect the spread part, and can simply stack the box and / or play short zone.
We saw this during the first third of the season with Brandon Wimbush at the helm, and all of 2017 as well. While we were able to get away with pounding the ball up the middle against the weaker teams, we certainly got badly exposed against Miami (they were sitting in single coverage, daring us to try a short pass), and would have gotten squashed against LSU if Book didn't come in.
Jalen Hurts is a fine runner, but isn't that much more accurate than Brandon Wimbush was. We'd basically be looking at a repeat of 2017 at best.