The champions received a blow early in the first half when Argentinian forward Sergio Aguero hobbled to the sideline after sustaining a knee injury. In the 18th minute, David Silva failed to convert a penalty.
In the second half, Silva had a chance to increase his team's lead in the 55th minute but he only managed to hit the bar. Soon Southampton drew level. In the 59th minute, Joe Hart failed to block Richard Lambert's shot.
With another substitute Steven Davis scoring in the 68th minute, the onus was suddenly on Roberto Mancini's side to restore parity.
In the 72nd minute, Edin Dzeko did exactly that. Seven minutes later, Samir Nasri made the home fans happy by scoring City's third goal.
Nigel Adkins had Billy Sharp replace Jason Puncheon in the 85th minute but this time the move did not pay off. Even though Southampton got an additional three minutes in the second half, they could not find the equalizer.
Despite City extending their unbeaten home run, Mancini had more reason than Adkins to worry after the first game of the new season. The fact that Aguero had to leave the field on a stretcher suggests that his injury is serious. Whether he will play the next game remains to be seen.