Colback does not feature in Benitez's plans and spent the second half of the last campaign on loan at Nottingham Forest, with 16 starts being made in the Championship.
Forest are allegedly keen on signing the 28-year-old on a permanent deal, and Benitez has acknowledged that the player is among a group who have no future at St James' Park.
The Spaniard told Chronicle Live: "The other players, they know because I told them I wanted to keep a smaller squad with players who will be part of the future. The others know they have to find a solution.
"It's the same situation [with Colback]. He's already talking with a club and then I think he will find a solution."
Colback has not made a first-team appearance for Newcastle since May 2017.