The pair both broke metatarsals last month on international duty ahead of the opening Champions League game in Porto, and were both told it could be six weeks before they are fit again.
"We will now have a two-week break for the internationals and we will see how they are after that.
"But even after that, it may not be quite the time. It could be another week or so before they can be considered after that."
But Alonso, who has been badly missed in midfield, is ahead of schedule.
Said Benitez: "We've got to be careful and we'll have to wait and see about Xabi.
"I think he has a better chance than Agger of being back next week.
"It is too early to say anything at the moment. They are both progressing well but I'll need to speak with the physio and the doctors next week before we know what the situation is."