Here are some typical answers(ranked in ascending order of corniness) I get from managers/bosses whenever I bring up the importance of having unit tests and code coverage as
Don't sell management on a particular approach; that's just going to be difficult and isn't really going to buy you much. Whether or not your management chain appreciates unit tested code doesn't matter.
Sure, unit testing your code has a lot of benefits associated with it, but don't rely on management buy-off to write your tests. When people start seeing results, they'll flock towards The Right Thing.