He was used sparingly against Salzburg and Wigan and then this week, when Mancini picked so many youngsters for the Carling Cup tie at West Brom, Johnson was in from the start.
And that bad news would come with a little nudge from Mancini for Johnson not to get side-tracked by his new-found fame.
"Adam has to understand that sometimes it is important for a player to play for a team," said Mancini. "He can have a top career if he understands that he must work and think about football every day, not about other things.
"I am very happy that he played for England but he needs to keep his feet on the ground. That is important, not just for Adam but for all young players. It is easy for a player like Adam, with a good attitude and good quality, to arrive at the top. But you must always stay there. You can never go back.
"Sometimes when players arrive at the top they think they have finished. Actually, you must work better than before."
Mancini knows what he is talking about.
As a young player himself, the Italian concedes he did not have the best attitude and it took two managers to focus his mind.
"When I started my career I didn't work very well," he said. "I didn't work hard and thought if you had enough quality you could play for the top squad. But if you don't work you can't improve.
"Vujadin Boskov changed my mind and I also worked with Sven-Goran Eriksson for many years. They just sat me down and told me what to do."