The two clubs have been drawn in the same Champions League group.
Redknapp studies Benitez's task in Italy and wonders if he has the impossible job following Jose Mourinho's Treble-winning season, which ended with a European Cup triumph in May.
He told the Daily Express: "It's like following Fergie at Manchester United. What more can you do? But then Rafa obviously believes in himself and that he can do something big there.
"They are the holders, but really the Spanish clubs are the ones who it will all be about. I backed Inter to win last year simply because Jose Mourinho was there. Now he's at Real with so many great attacking players like Cristiano Ronaldo. And Barcelona spent big to get David Villa before the World Cup.
"For many years I've always taken English clubs to be dominant. But if any of them get beyond the Spanish teams this season, it will be a fantastic achievement."