The first game of the Benitez era takes place at the King Power Stadium tonight as the 19th-placed Magpies travel away to league leaders Leicester City.
It has been a horrific season for Newcastle, who shipped 53 goals under former boss Steve McClaren before his sacking on Friday morning.
Benitez's side, who have 10 games left, one more than several of their rivals, are one point behind Sunderland in 17th, but their former goalkeeper Hislop claims that without a solid defence, they risk being relegated for the second time in seven years.
"He's got to start at the back. The defence hasn't stood up to the test as it did the last time Newcastle were relegated," he said on Sky Sports.
"Right now, with nine games to go, it is a quick fix and you're hoping there is a reaction to the new manager coming in."
Tonight's clash at Leicester is big, but Sunday's visit of the Black Cats is massive for both relegation-threatened sides and it will be Benitez's first game at St James's Park.
Two defeats in both of those games and Benitez would do well to lift his side's morale for the remainder of the season. The Spaniard, who won the Champions League at Liverpool, signed a three-year deal.