The 19-year-old is a product of the club's famous youth academy and has featured at right-back throughout pre-season.
"This is Harry's time," Gradi told the club's official website. "We never really regarded him as one of our high flyers when he was younger but he was always a steady player coming through.
"He has stuck at it, improved and as one of our long-serving kids we don't have to teach him much.
"He knows what he has to do and I don't think he will be totally different from what Mat Mitchel-King did for us last season.
"If Mitchel-King and [Patrick] Ada can play at full-back then I am sure he can as well. He won't roar forward like Matt Tootle does but he will defend well and can offer us height at set-plays."