I think having to add an option in the game setup on what to do with a partially injured player kills this idea.
Its a nice idea, i just tihnk it needs to be looked at from a slightly different angle.
As mentioned, game shape does 'simulate' a players lack of ability after he has time off, and frankly game shape is hard enough as is to manage without having to deal with partial injures.
At the moment, injuries, overtime, grabage time, fouls, are all in contriubtion to the frustrations of managing game shape. These partial injuries would just make it so annoying.
and currently a player can get injured in a game, sit out that game, but then he isnt injured for any future games, so i would kind of call this a partial injury anyways.