The former Liverpool midfielder has been previewing one of the biggest matches in world football.
The Reds signed the 24-year-old for a £29 million fee this summer, as reported by BBC Sport, and he came good for Jurgen Klopp's side against Arsenal on Wednesday night - scoring two fine goals, his second the pick of the bunch.
Firmino has been criticised for a trait fellow Brazilian Philippe Coutinho has shown - inconsistency.
But Redknapp feels Wednesday marked the forward's true arrival in English football, backing him to shine against Manchester United and regularly thereafter.
The pundit told Sky Sports: "I predict Liverpool will just come out on top and secure an important win for Klopp and the development of his team.
"I think this week we saw the moment Roberto Firmino arrived. Against Arsenal, he showed exactly what he can be. He was phenomenal - and proved he can do it at the top level. Like most foreign players, he's needed time to settle in but he will be key on Sunday, now."
Liverpool fans will be excited to see a talented Brazilian at his best at Anfield, especially if Coutinho can raise his game once again and combine brilliantly with Firmino, too.
The only hole in Redknapp's theory, though, is the fact the 24-year-old shone against Manchester City but then disappeared for some weeks afterward. Will he shine against Manchester United?
And will Klopp's men rub salt into Louis van Gaal's wounds?