Pep Guardiola believes that John Stones has not fulfilled his potential during his time at Manchester City.
Moreover, injury concerns have hampered his progression, the latest being a hamstring problem picked up against Manchester United that kept him sidelined for a month.
Speaking on the subject of his defender, Guardiola told Sky Sports News: "He has the conditions to do it, but to be the best central defender you have to play one entire season, every three days being there. It's difficult to reach the potential that he has, he has to be at the level to play regularly. The last three seasons and a half he could not do it.
"Sometimes when you have a tough injury - for example like Aymeric [Laporte] had this season or [Ilkay] Gundogan in the first, or [Benjamin] Mendy the year he arrived - it's unlucky. But in the case of John, I don't know why he has not been able to play regularly.
"We try to help him. We have tried to do the best to help him since the first day of the three-and-a-half years we have been here. We do everything for him. It depends on him. It's in his hands and in his head to become one of the best."
Signed for £47.5m from Everton in 2016, Stones has not played five consecutive games for over a year.
The defender returned to action for the Citizens during a 4-1 win over Port Vale in the FA Cup on Saturday.