I do wonder is that the case also ^^ I know on some other football management games it is. They'll go from being out for 3 days to 10! because you left them on and their condition deteriorated. [pretty sure Football Manager does this] I say get them off ASAP, even if you just use one of your weaker subs to fill in. Anything is better than having no active player in the position, and also risking a longer term injury, if he is a valuable player.