Do we need interfaces for dependency injection?

Question

I have an ASP.NET Core application. The application has few helper classes that does some work. Each class has different signature method. I see lot of .net core examples on

Answer 1

Does it work? Yes. Should you do it? No.

Dependency Injection is a tool for the principle of Dependency Inversion : https://en.wikipedia.org/wiki/Dependency_inversion_principle

Or as it's described in SOLID

one should “depend upon abstractions, [not] concretions."

You can just inject concrete classes all over the place and it will work. But it's not what DI was designed to achieve.