The Magpies have made just five signings during the summer but Benitez says he must trim his squad in order to bring in some new players, as they prepare for their return to the Premier League against Tottenham on Sunday, live on Sky Sports.
Owner Mike Ashley had promised to give Benitez "every penny the club generates" from promotion and players sales to rebuild the squad.
But the Spaniard confessed to being frustrated over the lack of incomings and outgoings at the club last month, and following injury to striker Dwight Gayle, is desperate to improve his team.
We know what we need and we will try to do this until the end of the transfer window," Benitez said following Newcastle's 2-0 win over Hellas Verona on Sunday.
However, Benitez stopped short of naming how many transfers he is targeting.
"We are trying to move players out to bring players in - that is what we are trying to do," he added.
"We know where we are and what we need and if we can get that, I'm not sure."
