Fan clutch springy thing?

[300decel]

My water pump replacement went OK, thought I had all bits ordered. This afternoon I did a Diesel Purge* and while the return fluid was clear, the idle sounds better to me.
When I shut the car off I heard a "ping" on the concrete garage floor and found the flat piece in the pic.
Is it vital to the fans operation? If I recall it was on the front of the fan clutch.
I would not like to pull the radiator to try and reinstall it unless it's really necessary.