> I'd like to tell them this in a productive manner ...
First, have 2-3 possible SOLUTIONS to this problem when you have the conversation with your Boss.
(Don't make it his problem.)
Frame the conversation in terms of your professional strengths.
We all should be thinking about the Highest & Best use of our time.
If you weren't working on this Assigned Task -- what are you willing/able to do?
Team Player Mindset: What would add more VALUE to the organization?