I tend to play my subs/reserves in the starting line-up if I am playing in a simple cup tie (early stages of the cup) or if im playing in a game I consider to be a dead rubber - like champions league group stage last game and already qualified for next round or the league final fixtures and the title has been won and theres no unbeaten record still intact.
I tend to keep 20 players in the squad, 2 goalies, 6 defenders/ 8 midfielders or 7 defenders/7 midfielders and 4 strikers. So injuries are normally covered. Any major injuries - i.e. key squad player then thats when the medic packs come into play.