I had my lower ball joint changed and the mechanic forgot to re-install
the knuckle ring.

Is it important?

Here is a picture of it, I matched it to what appears to be the knuckle ring from the Helms manual.

 

I think it'll be fine. I left one side off for several thousand miles and nothing catastrophic happened. The knuckle ring is probably a seal for the back side of the wheel bearing, and it would take a while for it to damage anything.

 

Thats what I figured, so it must be sealing the outer race but I think the CV joint would seal it plenty.

 

i believe that's just a dust shield for the cv axle.
on 90-93, it's on the knuckle. on 94-up, it's on the axle