The Reds pair are currently on international duty with England and Johnson says his team mate will cause problems for defenders at any level.
"He scares the life out of opposition defences," said the full-back.
"Everyone sees Andy as a big target man who can hold the ball up. But there is far more to his game than that.
"He is always an outlet. You can stretch the play by giving it to him and he scores goals. Defenders just do not like the way he plays.
"He has caught me by surprise with his development but in football your life can change very quickly.
"There is no doubt he can make a big impact."