In my current project we are using Dependency injection and are trying to depend on abstractions instead of implementations as much as possible. Someone recently added a static method call to resolve mappings of properties between a DTO and an Entity. My first reaction to this was that it should not be static. When asked why, I stated that if the mapping was static then wherever it is called I have to know what it is doing in order to unit test code that contains calls to it. When I heard myself say this it made me think, if it does not impact my ability to unit test code that calls it maybe it’s ok? I still think it is better to not directly couple the mapping implementation to callers, but if it does not impede the ability to unit test the callers then really it is more of standards question. So, in light of that if your standard isn’t to inject all dependencies and a dependency does not cause testing difficulty then I suppose it is ok.
That said I think there is value in sticking to standards so that readers of your code and other developers have a reasonable expectation of what they’ll find in a given circumstance. That is debatable, however.