Former Everton boss Roberto Martinez believes John Stones still needs to improve 'many aspects of his play' if he is to fulfill his potential at Manchester City.
Stones has come under fire this season following a number of underwhelming performances for Pep Guardiola's side.
The England international joined City from Everton for £47.5million earlier this year - making him the second most expensive defender in history.
However, he has struggled to adapt to life at the Etihad and made a series of costly errors during his side's 4-2 defeat to Leicester last week.