Redknapp worked with Di Canio at West Ham United.
He said: "It's amazing how suddenly he's a fascist now he's at Sunderland and wasn't at Swindon.
"No one mentioned anything about him when he was at Swindon, suddenly he goes to Sunderland and all the dirt in the world gets dug up on him.
"I'd love him to do great at Sunderland, I really would. He was a great player for me, a good lad, a great trainer, the fittest guy you would ever wish to see.
"He will want his players to be the same, to live right, conduct themselves and eat right and be as fit as they could ever be. He will demand that from them.
"Like I said, no one started on him when he was at Swindon, as soon as he goes to Sunderland people start digging things up about his past.
"He was loved at West Ham, he was absolutely idolised by the fans. He was a fantastic player.
"I don't know (if he will keep them up). He has a tough job on his hands, but he will give it his best shot."