Benitez is considering Greece international full-back Vasil Torosidis as a replacement for Glen Johnson, who could miss up to two months with his knee injury.
Asked if he was considering signing a full-back, Benitez said: "Yes, we are working hard (on transfers) and trying to do our jobs, and we'll see if we can do something.
"We are looking in the market but it's not easy to find good players available because the majority of them are playing in top sides. But we are trying to find solutions if we can.
"We have some good players who people are interested in because they're not playing too many games for us, but we have to manage carefully."