It's not about the kicker not working out. It's about seeing an issue with the rookie and not seeming to address it. The kid was constantly kicking a low ball. Either the kid was not taking to coaching (not listening) or the coach was not working with him. If he wasn't taking to coaching he should have been canned earlier than they did it. Add to that the issue he had in a game when he didn't kick off according to the wants of the coach (think it was the whiner game). Add to that his seemingly odd preference of putting in Cooper to PR when Webster had been doing the duty adequately before the half in the last game.
Add to that the poor showing on KR coverage all season, save for the last game (because the returner was returning every kick including those 5 yards deep in the end zone).
