Well, its kind of like saying, that no youth player will ever get a chance (except some crazy injury crisis hits the team). Iheanacho has Aguero and Bony ahead of him with the option to play sterling in the position. 2 or 3 key players have to be injured, that he´d get a run of games and it is probably not ideal to introduce a youngster to the team, when too many key players are out. Personally I think, that City and Chelsea have two of the most talented youth teams in the world (far more talented than United´s youngsters). It is madness, that they are unable to make room for any of those guys.
It's different though, Lopes is competing for a position that City are already well-stocked in and almost certainly the strongest area in the squad. City could have easily signed another striker to replace Dzeko and Jovetic but they didn't, they put faith in Iheanacho and he will be given chances this season, he's been on the bench for all the games and came on against Watford. Once the CL is underway and the cup competitions the games start coming thick and fast and rotation will become necessary. If Lopes was a CM or a full-back we would not have sold him, for example, as those are positions that need addressing in the near future.