I thought I had read that a player could not return from injury until his health reached 60, but I have an injured player with health at 58 and available to be used. So, two questions...

1. If I do play him, I have to expect a decrease in ability/effectiveness, right?
2. Aside from that, what are the benefits to resting instead of playing? Does it speed recovery?

1) Yes
2) If you don't play him it is possible (depending on promises or class level and minutes previously played) that he may start bitchin" about minutes.

Allegedly playing him at diminished health does not seem to make him more inclined for another injury.
That is my understanding.