City's £38 million signing from Atletico Madrid scored twice and set up another after climbing off the bench in the 4-0 defeat of Swansea on Monday.
It was a perfect start to the season for the team managed by Mancini, who is determined to follow up last season's FA Cup success by making a serious title challenge.
City finished nine points adrift of champions Manchester United last season.
But Aguero's performance suggests that they could be a real threat this time, particularly as Mancini believes Aguero is not yet 100 per cent fit.
"I know Aguero very well, but I don't think he is 100 percent," said the Italian, who also saw David Silva and Edin Dzeko score in his side's first game of the season.
"He has worked with the team only one week and maybe he needs to improve.
"But it's important that on his debut he scored two goals. He has scored a lot of goals in Spain and Argentina. He is a fantastic striker, but he played in Copa America not so long ago and he needs to do more training."
Aguero could line up with compatriot Carlos Tevez, who was not considered fit enough to face Swansea, at Bolton on Sunday.
Mancini is warning fans not to get carried away - despite his team's flying start.
"It was important for us to start the season well," he added. "But it is going to be a really long and hard season."
"Swansea played really well in the first half and it was easier for us after the first goal. Before then we found it hard."
Swansea are playing in the top-flight for the first time in 28 years.
And manager Brendan Rodgers insists he saw enough positives from his side to suggest they can stay up - despite suffering a painful lesson at the hands of big-spending City.
"I thought the result was harsh on our team," said Rodgers. "It might seem strange considering the result, but I have congratulated my players on their performance.
"We could have defended the first three goals better, but the fourth goal was a world class goal from a world class player.
"It was a wonderful cameo from Aguero. His two goals show you what he is about. But I want to pay tribute to my own players.
"Some of my players have played in League Two, so to stand up to a team of Manchester City's quality for so long is a credit. I know we will improve. We did enough to show that we can be a real threat in this league."
Source: AFP
Source: AFP