The Partenopei will face the Portuguese giants in the last 16 of the Europa League after eliminating Swansea on Thursday with a 3-1 aggregate win.
The Partenopei reached the next round of the competition with a 3-1 win against Swansea at the Stadio San Paolo on Thursday on the back of a 0-0 first leg draw at the Liberty Stadium.
Porto, meanwhile, followed up a 2-2 draw at home with a 3-3 stalemate at Eintracht Frankfurt this week, with a strike from Nabil Ghilas four minutes from time sealing Paulo Fonseca's men's progress on away goals.
Benitez, who guided Chelsea to Europa League success last season, was hesitant regarding his team's chances of making the quarterfinals and is especially concerned with the leg at the Estadio do Dragao.
"Porto are a very strong team," the ex-Inter and Liverpool trainer told Sky Sport. "The market in Portugal is very different, there are 200 Brazilians who cannot wait to land there.
"The environment will be very difficult to deal with when we go to their stadium, it will be a very difficult test."
Napoli, which sits third in Serie A, plays its next match at Livorno on Sunday.