Hi friend,
I have got the best answer for you.I hope you like it.

As you know injuries means that player is out of squad for given days.So the player is just out of matches not from your squad.You can't train him so condition is only given to those players who are there to spend condition by training or matches
So no condition will be rewarded if your player is not fit for losing condition by training or matches.

They lose condition as to show impact of injury.As injured player needs rest so you have to spend rest pack on him.