Ironically that's why Holgate can improve more than Stones can as he's under the watchful eye of Koeman, someone who can develope his defensive capabilities. Stones is under one of the best attacking managers in the world who wanted him for his on the ball, attacking capabilities so what's he going to learn , how to be even better at playing out from the back, whereas he probably needs someone like Allardyce to hone his defensive play.On the ball he is already one of the best in the world at the back, and his last ditch tackles are excellent too but he has a lot to do to fulfill the potential a lot have seen of him. I hope he does do well though and if he can concentrate on improving his weaknesses then City will have got themselves a very good player.
Holgate is also likely get even more benefit from one of the best managers when it comes to improving defensive talents of young players - Unsy.