He had a whole season. At one point he actually seemed to be going backward. with training and a whole pre-season i'd say the club know by now if they have a decent player on their hands, or a bit of a flop...at stuttgart he was dire, as were others to be fair, but there was no pressure in that game really.
What YOU are suggesting is that city stick by a player for an extended amount of time in the chance he might come good...that is all well and good when you have a very dependable set of other defensive options, but city don't have that luxury at this time unfortunately. Vinny was terrible last season, demi is getting old and wishes for his move soon, denayer is unproven, and that is about your lot.
Bringing in a CB with experience, and a good recent track record is very important for city's title chances imo...Besides if it ends up being a loan deal, then mangala can develop a little more in spain, and possibly return a better player.
Either way though in the short term city will be better off.
This, succinctly put.