Nobody in their right mind can write off Guidetti from the information they have so far seen. They can gamble on him not making it at City but that's what it is, guesswork.
For people to suggest that scoring at an alarming rate in Scotland & Holland means nothing, is just fucking stupidity. If he'd just held down a place & done 'ok' it would mean little , but the fact that he keeps scoring means that he is a real force at that level.
The next logical step would of course be to try him at the top level & find out if he could have a future. If, after a decent chance, he can't make an impression, & more importantly, doesn't score, then it's likely he is only fit for a lower level.
If circumstances prevent that at City, ie he leaves, then it can't be helped. If they just let him go without finding out, they are just purely gambling.