City started their Premier League title defence in the same way they won the crown last season as Mancini's men recovered from 2-1 down to win in a frantic encounter at Eastlands.
But the victory, which bore more than a passing resemblance to that epic 3-2 triumph against QPR on the final day of last season, was marred by the injury to Aguero, who twisted his knee following a fair challenge from Southampton defender Nathanial Clyne early in the contest.
Aguero was stretchered off the field and taken to hospital for scans on the injury amid fears of ligament damage, with results expected to take up to 48 hours.
With only 10 days remaining in the transfer window, Mancini admitted that his final activities in the market could well be dictated by the results of those tests.
"We don't know at the moment, we need to wait maybe two days to get the scans," said Mancini. "We hope it's not serious, but at this stage I don't know.
"The knee is always dangerous and the injury was so strange. I am worried, yes, but we don't know yet.
"At the moment, we can say nothing. This can happen in every game, it is clear that you can be lucky through the year, but today it is a problem.
"If it is a serious injury, we will probably change our plans. Maybe he is out for a week, or three or four weeks, or after the international break."
Mancini had been keen on trying to sign Robin van Persie before the Arsenal forward chose City's title rivals Manchester United.
Now the Italian will turn his attention to a list of attacking targets including Swansea's Scott Sinclair, Edinson Cavani at Napoli and Fernando Llorente of Athletic Bilbao.
Mancini confirmed one piece of business that is close to completion is the potential sale of winger Adam Johnson - either in a permanent