Advocaat had initially pencilled the "physically fit" winger into his squad for Saturday's draw at Stoke after a training session the previous morning.
He confirmed Johnson's inclusion at a lunchtime press conference on Friday, after the club decided not to suspend him despite being charged with sex offences against a 15-year-old girl.
Asked if Johnson did not travel to the Potteries because he was "not mentally right," Advocaat said: "Yes."
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