I have researched this a bit and have not found the right translation. The closest I've got was "managers di assunzione" in plural or "dirigenti per l'assunzione".
Hiring managers or line managers are technical and administrative managers (not HR) also responsible for making the final decision to hire a job candidate. They belong to the "hiring personnel" supergroup and may delegate the tedious task of short-listing candidates to HR, head hunters or recruiters. Hiring managers are the future "would be" your boss basically.
If you'd like to see this in context, take a look at the project video here and enable Italian subtitles: http://tinyurl.com/y4erjs3z
I have used "managers di assunzione" there because I couldn't find a better alternative.
A professional translator wrote it like this:
Hiring managers, you don't have the time to linearly read through dozens of CVs
Assumere manager di cui non avete il tempo di informarvi tra dozzine di CV
assumere is a verb and not a noun so it makes no sense to me.
And the other context where is used is here:
Hiring managers, you’re on the other end.
Assumendo manager, avete il coltello dalla parte del manico.
assumendo is a verb and not a noun.