Lukaku has netted seven times in his first seven Premier League games for Manchester United since his mega-money summer switch from Everton.
He faces Liverpool for the first time as a United player tomorrow when they visit Anfield in the opening game of the weekend (12.30pm).
Jurgen Klopp’s defence have come under severe criticism this season, keeping just two clean sheets from their 12 matches in all competitions.
Redknapp thinks if United turn up to attack Liverpool, they can exploit a huge weakness they have.
“If I were Man United and I was Romelu Lukaku, I’d be licking my lips at playing against that Liverpool back four,” he said on Sky Sports.
“They are there for the taking, they are making mistakes, and Jurgen Klopp knows it.
“He’s changed the goalkeeper, he changes the back four regularly.
“I don’t think he knows the best team at the moment and that’s why he tried to sign a centre-back in the summer.
“He knows they have a huge weakness there and I think it is there for Manchester United to exploit if they want to.
“That’s the problem, will Jose (Mourinho) go there and try to be defensive and try to stop Liverpool playing their own game?
“Or will he go ‘we’ve got good players, we can win this one’?”