Johnson scored the equaliser and created the second goal for Samir Nasri as City booked their place in the quarter-finals with five goals in a 27 minute spell.
He said: "Sometimes he thinks, 'Okay, in this game I scored one goal, I did an assist, that is enough.'
"I think Adam, because he is young, can improve a lot. He needs to think in a different way.
"He needs to think, 'I scored one goal, I did one assist, I should continue because I want to score another goal and another assist, and run back to defend'.
"He can do this if he wants. My opinion is this, he has everything to become one of the top wingers."
Mancini added: "Adam is young, he is in his third season in the Premier League. I am disappointed when he doesn't put everything on the pitch.
"Tonight he played well, but for the second goal Wolves scored, he never followed his opponent.
"Adam is the only one winger we have in the team.
"He cannot play every game, it is impossible, but if he wants to improve, he can have a lot of chances to play."