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.
But Mancini believes the former Middlesbrough player can still do better and was critical of his role in Wolves' second from Jamie O'Hara.
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."
Mancini laughed off suggestions that unused substitute Mario Balotelli had been involved in a row with a member of his coaching staff after the game.
When asked about a possible incident, Mancini only said: "Who Mario? No, no. For a fire on the bench!"
The media were not permitted to ask questions about striker Carlos Tevez who has been fined four weeks' wages by City.
Edin Dzeko's brace and an own goal from Dorus de Vries completed City's haul with Nenad Milijas and Jamie O'Hara replying for Wolves whose manager Mick McCarthy took consolation from their performance.
He said: "That was an object lesson in finishing. But I was pleased with our performance.
"That sounds crazy when you've just been beaten by five, but I was happy.
"We started well, and deserved the lead. The fact we didn't hang on to the lead was nothing to do with defending badly.
"They [City] are all top players. They won't be made to work harder than they were tonight, that's for sure."