The Partenopei coach is interested in signing the midfielder but admits that a deal hinges on whether new Red Devils manager Louis van Gaal wants to keep the former Everton ace.
The Serie A club has been heavily linked with a move for the midfielder, who joined the Red Devils from Everton for 34 million euros last summer.
MORE: Biggest summer transfers | Soccer's beautiful fans | WAGs
New United boss Louis van Gaal has revealed that he will speak personally to those players he has deemed surplus to requirements and Benitez is now hoping for a "positive change" in Fellaini's situation following the Old Trafford outfit's return from its preseason tour of the United States.
"Fellaini is a Manchester United player - it does not depend on us but it depends on them," the Napoli boss told the press. "He is a great player but he is at another team. It appears he is waiting for Van Gaal. We will have to wait.
"Is he the right player? We have players who work very well, while it also depends on who leaves. It is not easy to name names. What features in a midfielder am I after? Players who have the mindset to win every game, then we can start to talk about the technical quality. With our same mentality but with a little more quality."
Fellaini made 16 Premier League appearances for United last season but failed to score, as he became a source of frustration among many fans at Old Trafford.