Redknapp is steeling himself for an afternoon of abuse in his first game back at Fratton since leaving for Tottenham a year ago next Sunday.
But Hart says: "Our crowd doesn't need a lot of encouraging and I am sure there will be a bit of spice behind Harry's return.
"But they have to remember what has been achieved at this football club over the last two or three years.
"What has happened has been unprecedented and there is only one fellow behind that. That's Harry's legacy to Portsmouth.
"He's a hard act for any manager to follow, especially with the change in circumstances here. But everyone will agree getting to Wembley and Europe was a massive achievement.
"Supporters have always got on the back of someone from the opposition.
"But I would like our supporters to get behind us and concentrate on our team."