Johnson has caught the eye of Capello since joining City in January from Middlesbrough.
Mancini told City's website: "Capello likes Adam, we know that. But Capello must decide between certain players, and it is very difficult. He has important players, and players with more experience than Adam. I don't know what he will decide.
"I think that Capello is a top manager and he knows very well what Adam can do, as well as the other players he has to pick from. It's not my decision whether he goes to the World Cup.
"But Adam can play right or left - the right side for him now is normal! He must still improve and keep thinking when he is playing, and he knows this.
"Adam is already a good player and he is also young. If he wants to improve, then he must keep working very hard. But he is a future England player for sure."