Only Michu and Kalidou Koulibaly have come in for Benitez's men, who have been linked to Manchester United midfielder Marouane Fellaini.
Speaking after his team's 2-1 friendly loss to Paris Saint-Germain on Monday, Benitez said there was plenty of work to do before the transfer window closes.
"There are a lot of names on the list, but we are where we are and the coach has to think about preparing his team," he said.
"We have tried to do something until the end but we haven't had time. We will continue to work in the next days."
Napoli's first pre-season loss came after second-half goals from Zlatan Ibrahimovic and Javier Pastore at the Stadio San Paolo.
Benitez believes his team have improved ahead of their UEFA Champions League play-off first leg against Athletic Bilbao on August 19.
"The team worked well both in attack and defence," the Spaniard said.
"We played a top-level opponent but we showed we have made a step forward. We have players who can make the difference, I have faith."
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