The former Norwich man endured a frustrating season at Sixfields which saw him score 10 times but struggle with an ongoing knee problem.
Johnson said: "Everyone has their favourites but it is my job to get a promotion winning team together and I have to make these hard decisions.
"In Leon's case everyone knows about his injuries and you can't guarantee that he will play enough games.
"If he is doing well you want him to stay in the team but he can't do that.
"He can't train every day so he struggles with the fitness side of it as well."
Source: PA
Source: PA