Barry believes Johnson, who moved to City from Middlesbrough during the January transfer window, can climb to great heights in the game.
The midfielder said: "Adam joined us in the winter with little experience at this level and he just keeps growing in stature. For the future, he can get better and better. His confidence keeps growing.
"The more he plays, the better he will get. Hopefully he will get more (game) time and the goals will keep coming."