How to inject private field into spy?

I have following class:

public class TestedClass{
   private List<String> strings;
   .... 
}

Test:

public class TestedClassTest{

  @Spy
  TestedClass testedClass = new TestedClass();

}

I know that exists annotation @InjectMocks but it works only for mocks but I want to inject real List

P.S.

List have not setter. In real code setter is redundant.