Cooldown indication, transparent rotating texture

Hi Unity fokes, I have a question. I am currently in the middle of developing a cooldown system, for a game which utilizes an interface similar to WoW, LoL, Rift etc.

The idea is, when an ability is used, the button becomes significant dimmer. As the ability slowly begins to cool off, the original image also slowly appears in a circular form.

I just don't know how I should go around with this. The only thing I can figure out is, that I should probably use a transparent grey texture, but I am in doubt on how to make the original image appear in a circular shape. Maybe I am forgetting something very simple and vital.

I hope you guys can help, best regards: - Marc