Agüero's second is an early contender for goal of the season, struck from 30 yards, as the FA Cup winners beat Swansea City 4-0 with all the goals coming in the second half.
"I'm not surprised because I know him well," Mancini said. "He's a fantastic striker and I still don't think he's 100% yet because he has been training with us for only 10 days. He can improve but it was still important for him to score on his debut. He needs to do more training and he is still recovering from playing in the Copa América but I am really happy because they were important goals."
Edin Dzeko had opened the scoring for Mancini's team before Agüero came off the bench in the 59th minute, turning in City's second goal from close range before setting up David Silva to make it 3-0 and, finally, hitting the superb strike for the fourth goal.
"He's decent, isn't he?" Joe Hart, the Manchester City goalkeeper, said. "He's come in at £38m, which is a big price tag, but it looks like £38m well spent."
Mancini has a formidable choice of forwards now that it seems Carlos Tevez will remain at the club until at least January. "We have Tevez, Mario [Balotelli], Agüero and Dzeko, and they are four important strikers," Mancini said. "There is no club willing to pay the value [for Tevez] and I think he will stay here."
Tevez, still officially the club captain, chose not to attend after being deemed short of match fitness. "I think he will be available for us for the next game," Mancini said. But on this evidence there is no guarantee he will be automatically in the starting line-up.
guardian.co.uk © Guardian News and Media Limited 2010