The Premier League champions looked to be in control following Carlos Tevez's opener, but goals from substitutes Rickie Lambert and Steven Davis in the second half gave the Etihad Stadium faithful a fright before Edin Dzeko and Samir Nasri earned City the three points.

City missed a number of chances to kill the game off earlier, most notably when David Silva saw a tame penalty saved by Kelvin Davis.

While Mancini was happy with the result, he insisted his side must be able to kill games off earlier if their title defence is to gather momentum.

"It's important we won the first game, it's always difficult," he told Sky Sports after the match.

"It's important to show character but it's better to close the game before, we had so many chances and when you don't close the game it can hurt you."

City earned crucial points last season on the back of some memorable come-backs, most notably in their 3-3 draw at home to Sunderland and their dramatic 3-2 win against Queens Park Rangers on the final day of the season.

And after watching his side come from behind to defeat Chelsea in the Community Shield a week ago, Mancini stressed that he hopes to avoid chasing results late on in games.

"It's not normal what happened last year, this happened two or three times," he said.

"I think today because it's the first game, we only worked for two weeks together, but we need to improve."

City's victory was tarnished by an injury to Sergio Aguero, who had to be stretchered off with a knee injury after just eight minutes and is set to undergo an MRI scan to ascertain the extent of the damage.

Mancini had no update on the striker's condition immediately after Sunday's match.

Speaking after the brave loss, Southampton manager Nigel Adkins said he was happy with his team's display against the champions.

"We've come away from home to take on the champions and we tried to find a way we could win this game," he said.

"That's what we tried to do and our defence have just faced the best forward line they've ever come up against and they didn't do too badly.

"City conceded only 12 goals at home last season and we've just scored twice. They only dropped two points at home and we pushed them hard today, so our lads can take a lot from this game.

"We scored two goals, Rickie Lambert, fantastic goal from him, it's a dream for him.

"For Rickie to come off the bench and score and then for Steven Davis to come off the bench, we've brought him in, he's been injured and for him to come off and score was really great.

"And what a great debut for James Ward-Prowse. The vision of the club is to get 50 percent of the youngsters through the academy and I think everyone at our academy should be so proud that at 17, young James made his debut today, I thought he was outstanding."