Balotelli, a £24million summer signing from Inter Milan, scored twice in the 3-0 victory against Salzburg in the Europa League, while Johnson was also on target.
The convincing victory means City have secured a place in the round of the last 32 with a game to spare.
On Balotelli and Johnson, Mancini said: "They could become among the best players in the world, strong players. They need to work hard all the time. They both scored fine goals but they must carry on."
Source: PA
Source: PA