Balotelli has yet to make a sustained impact for City since his 2010 arrival from Inter Milan and has not started a game in the Premier League so far this season.
However, he came off the bench to inspire City to a 2-0 success over Everton on Saturday and could be involved in Tuesday's Champions League clash against Bayern Munich.
Though the Italian often does not celebrate his goals, and can cut a moody figure on the bench, Milner believes there has been nothing unprofessional about Balotelli's attitude.
"I don't think anyone is happy when they are not in the team," said Milner. "But he hasn't shown it. He trains hard every day.
"He goes about his business like everyone else does. You try to get in the side and when you get a chance, try to take it.
"He has not stepped out of line or shown his disappointment in any way."
When Milner was told fans feel Balotelli is nuts but talented, he replied: "That is pretty accurate. He is a top-quality player though.
"You have seen those characters around the dressing room. You forget how young he is and he has done a lot in the game already. You can see his quality every time he comes on.
"He only needed half a chance and he took it. It wasn't an easy finish. He is a massive part of this squad as well. He is a goalscorer."
Meanwhile, City boss Roberto Mancini is confident that Balotelli has all the attributes required to become one of the world's top strikers if he can keep his mind focused on the job.
Mancini said: "With Mario it is never easy. But we keep in our mind that Mario is young, and young guys can have different behaviour.
"I know him very well. I know he is a fantastic guy.
"I want him to work well all the time, in every training session and every match because this year he could be an important player for us.
"He has everything. He has the technical ability. He is strong. He shoots very well. He can score with his head. He needs time but he can become one of the top players."
Source: Team Talk
Source: Team Talk