The Coppa Italia winners have been keen to strengthen in midfield this summer as they plot an assault on the Serie A title and have been linked with a host of central midfielders.
Fellaini was near the top of their wishlist but, as revealed by Goal in July, the move stalled due to the 26-year-old's wage demands, as he currently earns £100,000 per week with United.
And Benitez now fears that Napoli have missed out on their top targets, with Barca persuading Mascherano to stay and Fellaini tied to Old Trafford for another four years, and the Spanish boss is instead turning his attention to improving his current crop of players.
"[Mascherano] has two years left on his contract. [Barcelona] know he is an important player, they talked to him and made him feel more important," he told Sky Sport Italia.
"We must continue to work with these players and see if we can improve what we have.
"Fellaini? He has a contract with Manchester United, so we're thinking about the players we have."
Benitez previously admitted interest in Fellaini but said that the Belgium international was waiting to see what Louis van Gaal's plans were for him, with the Napoli coach's latest comments suggesting that there could yet be a role for the former Everton man under the Dutchman.