I don't think thats the case, Heskey got worse, Shorey got worse, Luke Young got worse, guys name who eludes me, came from Rangers, got worse, Davies got worse, Petrov became a far more defensive player and less good offensively, Sidwell was worse. He didn't improve Reo Coker, or really anyone else for that matter.
Milner had an incredibly average season, remove the penalties and its basically no better than any other season, frankly Milner just had a longer season with more cup games than usual, hence a few more goals and assists in total, World Cup + being english took care of the over stating how good he was. THe players who have played very well, Ashley Young, Carew, Collins, Laursen, were simply very good players when they arrived.
Saying O'neill got the best out of them, and no one else can, is just not close to accurate.
It won't be surprising to see them do worse though, unless a team is severely underperforming then new managers usually take time to adapt a team to their own methods, and if they are in a highly competitive area of the league(3-8th) then that will take its toll.
Sven will make them a much much worse team, he's crap, all his buys at City were short term form players, almost every last one of which has flopped and has been or is in the process of being shipped out, with most doing crap elsewhere now. Lets not forget how he also prevented a very good Mexico team qualifying for the World Cup which before him being hired was a virtual certainty, and he was pap and has been told to smeg off from the Ivory Coast aswell, his single spell of success, was when he spent £237mil to buy the league for a already 4th place Lazio.
Bob Bradley however had a not fantastic american squad and did have them performing way beyond everyones expectations, playing good, organised, strong, attacking and good to watch football, not sure why people are down on the guy.