The club's £38million record signing came off the bench to score twice and set up another as City opened their Barclays Premier League campaign with a 4-0 hammering of promoted Swansea.
"I know Aguero very well but I don't think he is 100%. He has worked for only one week with the team," said Mancini. "He needs to improve but he is a fantastic striker He played in the Copa America and he needs to do more in training, but I am happy because he scored two important goals."
Mancini was pleased to get a campaign in which his side are expected to challenge for the title off to a winning start.
He added: "It was the perfect start. It was important to start the season very well and it was important to score the goals and score with the strikers. But the season will be very long and very hard, every game is very difficult. In the first half Swansea played very well."
Mancini left controversial striker Mario Balotelli on the bench while wantaway Carlos Tevez was not in the squad. Despite that, Mancini insisted all his forwards would still have a role to play.
He added: "We have four important strikers and they can play together."
Swansea made a composed start but were ultimately outplayed by the hosts, who also hit the bar twice through Silva and Gareth Barry, but manager Brendan Rodgers is convinced the Championship play-off winners will make an impact in the top flight this term.
Rodgers said: "I thought overall the result - although there were some fantastic goals - was harsh on our team. For the first hour the players showed great confidence and great quality on the ball and we looked like we could be a real threat.
"We will get better and we want to improve. We understand now - like we did before the game - that if you make mistakes at this level you get punished. We showed we can be a threat in this league and we will take all the positives from this game and go again on Saturday."
Source: PA
Source: PA