Here's an idea:
The company (Bosses) could launch a strat plan to accomplish a specific goal, after conducting meetings, of course. And they would normally use the subordinates to do that (delegating). Now, the assignment might not be specifically included or stipulated in the original job description of the subordinate but it may be added as a directive by the boos/higher authority.
Now, here's the thing. If it is not in the job description, and the subordinate does not/was not able to do the assignment satisfactorily due to inadequacy, how may it affect the subordinate? The boss could put it in his evaluation. But it should not affect his tenure because it was not in the subordinate's job description to begin with, and in point of fact, it is the manager's accountability to begin with. If the company decides to cut you because of it, you may contend it through a grievance procedure.