In both cases I think you're wrong, De Jong has looked a shadow of his former self, because he's basically not played since the end of last season. Take Messi, don't play him for 6 months and ask him to be his best in his first game back, not going to happen. If you rotated him more he'd have more form, be fitter, sharper and less of a problem when you're forced to rely on him.
This is ultimately the situation, every team ALWAYS gets injuries, when you refuse to rest players, get an injury and fall back on an unfit player, you've made a mistake, even more so as Mancini KNEW Toure would be gone yet didn't help De Jong get games before there was no other choice.
15 minutes here, 30 minutes there, he should have been getting game time for a minimum of 4-5 games before Toure was gone, realistically longer than that.
Also the argument that you played Toure, are top of the league so it worked, is baseless, you haven't proven anything there. Its two unconnected statements. Don't forget that City's form has been significantly worse in the past month and their gap has gone down to literally nothing right now. I'd argue that both not resting Silva and Toure is the very reason the gap is shrinking, not the reason you're top. Toure has looked shattered for over a month, and Silva has been out of form for the same amount of time and you've dropped what, 10 points or so in that time frame while giving up a 6-8 point lead.
Likewise there were plenty of games you could have won without Toure this year, and ones you didn't win despite him playing recently where the team looked tired and out of idea's.
These things are all connected.
If De jong had started 5 more games spread over the season so far, and subbed in for Toure even time you were 3-0 giving him 30-40minute's less in another 5 games, then De Jong would have played more, Toure less, he'd have been better over the past month and De Jong would be better now.
Wait for a RVP injury, wait for our form to tank as we rely on Chamakh who has barely played and pretty much not started 2 games in a row for over a year now.
You rotate to avoid these problems, and its up to a manager to pick the right times to do that. Mancini hasn't done that at all so far, and that's come back to bite him in the ass now.
I saw a hilariously obtuse write up of City's not great performance and last loss where someone banged on about both how they said Yaya, Silva and Kompany were three key players City couldn't replace.......... and how City haven't looked really good since the Norwich thrashing coming up on 6 weeks ago.
They didn't spot the irony, yes City haven't looked good for 6 weeks, and its only the last game they didn't have Yaya, Kompany and Silva..... but they looked poor before then for 5 weeks the problem wasn't missing them, the problem was with two of them looking no where near their best, while not at their best and looking tired what did Mancini do, rest them, bring Yaya off early, nope, just play 90 mins every game and he looked more tired and worse every game.