The England winger cost £10million but, unlike fellow summer signing Steven Fletcher, Johnson has yet to really get going on Wearside.
The 25-year-old admits he needs to improve but is confident he and his fellow midfielders will soon start producing chances on a regular basis.
"I must work harder," he told The Sun.
"There are a lot of things we can do better. We haven't created that much going forward but hopefully that will change and we'll be able to score a few more goals.
"Sometimes it's the way teams set up against us because they know we're a threat, especially with the front four we've got and when me and James McClean stay wide.
"We've got Fletch plus Stephane Sessegnon and Louis Saha, who are proven quality goalscorers, but we need to create more chances.
"Me and James on the flanks - and the other midfielders - need to work harder for the strikers. Hopefully it will come."
Johnson also backed Irish winger James McClean to start reproducing the form which he showed last season.
"I remember when I was just coming through," he said.
"As wingers you can get frustrated. I could see a bit of that with him but he's strong enough."