It’s definitely good to know Zend or some other framework, and you could probably get by without knowing how it works under the hood, but you’d be a better programmer if you did.
As for whether it’s a deal-breaker if you don’t know Zend… that depends on each employer. For some, yes, it’ll be a deal-breaker. For most, I think, it’s enough to know at least one framework, to show that you’re familiar with the architectural concepts, design patterns, and coding practices that go along with frameworks.
Zend is simply one of numerous PHP frameworks out there.
I would simply get familiar with the concept of MVC and how it’s used, and be honest with the recruiter that you don’t know Zend FW, but you’re familiar with MVC and can pick up Zend FW.