Johnson has played seven times in all competitions since leaving Manchester City during the summer in search of regular first-team football.
The 25-year-old is yet to score for the Black Cats but believes that he is close to an improvement as he searches for form.
He told the club's official website: "I've had a slow start since moving to Sunderland.
"I haven't been too happy with my performances if I'm honest, but I'm now starting to feel that I'm lasting longer in games and getting fitter after my injury.
"I'm hoping to get better and keep on improving. Now we need to create more chances and go on to win games."