Mancini urges duo to kick on
Balotelli, a Â£24 million summer signing from Inter Milan, scored twice in the 3-0 victory against Salzburg in the Europa League, while Johnson was also on target as City 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."
Balotelli made the breakthrough with a stunning volley following a cross from Pablo Zabaleta.
He then extended City's lead by sweeping the ball home from close range after being set up by Patrick Vieira - one of eight changes in the side - with Johnson also having a hand in the move.
Balotelli, who did not celebrate either goal, was given a standing ovation when he came off in the latter stages of the match and was replaced by Emmanuel Adebayor.
Mancini explained: "That is just him. It is normal for him as he does it every day in training. He came off with a little knock.
"I think he can improve on that performance. He could run more and take more of his chances. He might have got two or three more goals."
Johnson rounded off the victory with a fine individual effort after leaving three dispirited Salzburg players in his wake.
He said: "I like coming inside on my right foot to try and get shots off, and luckily I got another goal. It was tough, in Europe it's slower than the Premier League and that showed but we got the result we wanted."
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