Benitez will promote Jack Hobbs to the senior squad as cover for Jamie Carragher and Sami Hyypia, but it promises to be an anxious period until Agger's return.
"We knew this was a position we could have most problems if we suffered injuries," said Benitez.
"We will be okay if Carra and Sami can stay fit, but we know if there are any more difficulties we will have more problems. Hopefully that will not be the case.
"Jack Hobbs will train with the first team and we will see how he progresses.
"Arbeloa also has experience at centre-half if we need him."