Fletcher has become an instant hit with the supporters and was yesterday named September's Premier League Player of the Month.
Johnson told The Journal: "Fletch as been unbelievable since he signed. Nobody expected him to be this good straight away.
"He's a goalscorer and that's what a team wants. If he doesn't do anything all game and scores a goal to win you the game, then that's enough for me.
"He wins us points. He's at the right place at the right time, and will continue scoring goals."