Whoever they hire needs time to teach his assistant coaches the offensive / defensive system he plans to install and the techniques / reads he wants taught. Ideally, this would need to be done BEFORE spring practice. To some degree, fundamentals are fundamentals, but techniques and reads vary some according to what type of system you're running. If all of the assistant coaches have to stand and watch, along with the players, while the new guy does all the teaching, spring practice (and even summer workouts) won't be as effective as they could be.