He told Liverpool's website: "You worry a little bit when you're going to be up against Ronaldo, [Carlos] Tevez and [Wayne] Rooney because they are fantastic players - but that's my job.
"We have big defenders here like Carra, Sami and Fabio. They are great defenders, so I can sleep well."
The fullback added: "Fernando [Torres] and Stevie [Gerrard] are playing really well, scoring goals and have a good partnership. I think the Manchester United defenders will be really worried."