US Edition 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.
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.
But Agüero's performance suggests that they could be a real threat this time, particularly as Mancini believes Agüero has yet to hit peak fitness.
Mancini said: "Sergio is a photocopy of Romario, they are the same player. He is not 100 per cent yet but he is going to be fantastic for us."
Romario scored over 1000 goals during a lengthy 24-year career which saw him don various shirts from Vasco de Gama to PSV Eindhoven and Barcelona.
It is not the first time Mancini has referenced both Romario and Agüero in the same sentence either. Two weeks ago the Italian said: "I hope he can score all the goals that Romario scored."
Bolton face City on Sunday with Mancini hinting that Carlos Tevez could return for the game but the Italian knows that with Agüero, he has a special talent in his squad.
"He is a fantastic striker," he added. "He needs another two or three weeks to be 100 per cent fit but he is going to be a fantastic player for us and I am delighted with the whole team's performance."
Meanwhile Agüero was delighted with his debut and said on Twitter: "Very happy with my debut for City. Hopefully the start of something big. Thank you to my team-mates and the fans for their support."