I think the first thing to observe is that Williams is not a poor player , in fact before he arrived at Goodison he had been for much of his career a top player both domestically and on the international stage. So his failure to find form since his arrival here is either one of management or a problem of an ageing athlete, the truth is it's probably a blend of the two.
Could Guardiola restore his fortunes? Well if he is the great manager his supporters claim , he should be able to ,its a managers job to instruct and motivate players and as I've illustrated the really great managers can raise a players game.
However since Guardiola has only ever managed teams already laden with superlative players, it's hard to know whether this important aspect of management is one in which he also excels. Maybe one day he will challenge himself by managing a lower ranked club and then we can better judge if he is a great manager or merely a very good one.