Fletcher, who along with Johnson joined the Black Cats during the summer transfer window, has scored five goals for the club already.
Looking ahead to future matches, England international Johnson has backed his Scottish counterpart to continue his recent form.
"Steven's been unbelievable since he signed. Nobody expected him to be this good straight away," he told the Northern Echo.
"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 that's enough for me. He wins points. He's in the right place at the right time. He will continue scoring goals."
Fletcher is the only player to have found the net for Sunderland in the Premier League.