I believe Stevens was hired because he's into advanced stats and the Celtics are one of those teams doing this, that and everything else with them.
Ainge wanted someone who could work with his group of people... he didn't hire a coach to work for him! That's a huge difference in how a lot of teams are run and if this works out for Boston the media will probably paint this hire as the "beginning" of the "changing of the guard" so to speak.
The Celtics are going to be bad. They are going to "lose the right way". They've hired a guy that can handle that and possibly come out the other side.
It's a bold move which may or may not be successful... but you have to give Ainge credit for it.