Right, I'm pretty sure I did not make this rule up but in 3rd ed I distinctly remember a rule that allowed you to retire a player (due to injury) from your roster and add him as an assistant coach.

When I looked for the rule in the 5th LRB I could not find it anywhere.

Does anyone know if it existed and/or where it went?

It makes little difference as everyone in our league thought it was a good idea anyway so we are using it as a house rule.