As I see it, the manual says a player who is overused is susceptible to a day-to-day injury.

A player who is VERY OVERUSED is just susceptible to injury (as is a WORN OUT).

I read the "susceptible to injury" for the VERY OVERUSED to mean a possible longer term injury than the day-to-day of an OVERUSED player.

Was your guy OVERUSED or VERY OVERUSED?

If just OVERUSED, then I agree it sounds strange to get a 9 game injury.