Mancini is worried Johnson's overnight success with England when he scored against Bulgaria and Switzerland may have gone to his head and says he must fight to get back into the team.
"Adam is a young player who can improve a lot," said the Italian. "I would like him to understand that sometimes it is important for a player to play for the team. I think he's a very important player for us, but at the same time he can improve a lot.
"I am very happy he played well for England, but, yes, I think he must keep his feet on the ground as you say here. Not just Adam, but all young players who sometimes think they have arrived at the top and it's finished.
"But when you arrived at the top it's very difficult. It's easy for a player like Adam, who has good quality to arrive at the top.
"But when you get there, you must stay there always. You can slip back and to prevent this he must work better than before."