He is here for an emergency if Brezovan gets injured.
He is kinda the 2nd Choice but Walker is being given the oppurtunity to gain some experience of a match day.
If something happens to Brezovan then Naisbitt will be thrown in if we are not safe as we don't want to ruin Walker for life by playing him in a pressure occasion and him buckling
He was only ever signed as an experienced back up, should it become neccessary. Poyet has said a few times that he wants to blood Walker, if he thinks he is ready, before the end of the season providing we're mathematically safe. Naisbitt was signed in case of injury to Brezovan and Poyet not feeling Walker is ready.