Colback spent last season on loan at Forest with the City Ground outfit keen to sign him on a full-time basis ahead of the new season.
Forest's boss Aitor Karanka said: “I know Jack and Jack knows me and the club. He was really important here last season.
“It is true that he is on our list.
“We have other players on the list, because he is not our player and we cannot say that he is yet.
“We have to find a solution to that as soon as possible.
“But if it is not Jack, then we will find another target. We are working to bring in another one or two. It depends.
“Now we know that we are an attractive team to come to.
“The main thing and the thing I want to have is the squad together as soon as possible."