The 29-year-old Brazilian completed his £30m Stamford Bridge return yesterday following two seasons at Paris St-Germain.
Redknapp told Sky Sports: "This is sensational, the deal you wouldn't have expected.
"It suggests to me that Conte is looking to get another centre-back in so he has the luxury of playing three at the back sometimes.
"It was a system he played with Italy and some clubs in his career.
"Obviously they have Kurt Zouma to come back in, they have John Terry and Gary Cahill.
"Is David Luiz talented? Absolutely. Yes, he makes bad decisions and is a bit rash at times, but when he's on his game, he's fantastic."