Redknapp said: “If you left Ronaldo one-on-one with the best right-back in the world, he would beat him.
“Ronaldo was walking past Emmanuel Eboue at Arsenal last Sunday. I’ve never seen anyone do that to Eboue before.
“It’s very difficult to stop him. You’ve got to get tight with him and stop him getting the ball – which is not easy. Our full-backs will need all the help they can get. You can’t afford to get isolated one-for-one with Ronaldo. You need to shut down the flanks.
“I want to fill up both sides of the pitch. Our new signing Lauren has an engine and knows how to defend. So he can help double up with Glen Johnson on the right, but it is an even bigger problem on the left for us.
“Our new left back Djimi Traore is cup-tied (after playing for Charlton in the previous round) and can’t play in this one.
“We still have a few options. David Thompson could come in and play in front of Noe Pamarot – if Pamarot is fit. But we’ll have a look at it. Hopefully, Noe will be OK.”
He added: “If I were putting together a list of the best players in the Premiership, Ronaldo would be in the top five, for sure.
“He is an amazing player who has got everything. He’s 6ft 1in and great in the air. He has terrific power with his shot, unbelievably quick feet and the ability to beat people.
“When he first came to England some people seemed to want to dig him out because he has ability and can do special things. He has done well against us the past couple of times, to be fair. He gave us big problems when they beat us 3-1 and he scored at Fratton Park last season.
“We have to improve on that or we will get nothing again this time.”