I dislike these arguments. It's a similar principle to having a brand new Lamborghini in the garage and refusing to drive it in case it gets a scratch. It's also a full, circular, paradox... You want David Silva to play more because he's better than Delph (for example) > If you play David Silva he might get injured > To stop David Silva getting injured let's play Delph > City play worse than they would have if Silva would have played > Back to square 1.
Play your BEST XI for EVERY game, if someone gets injured or suffers a drop in form, replace them with squad players. We shouldn't have a cup team and a league team, we have one team which is highly competitive and if you're not good enough you're not in it!