An unparalleled talent drain in the summer led to much doom and gloom around St Mary's, yet the response has been nothing short of astonishing.
Koeman's side are second in the Premier League after their best-ever start to a top-flight season and boast the tightest defence in English football.
That form has led to much discussion as to whether Saints can actually see off the traditional giants and qualify for the Champions League - a feat Koeman believes could become more realistic over the coming weeks.
"We try to play game by game and prepare ourselves in the best way for every game," the Dutchman said ahead of City's visit.
"We know our possibilities but we have played 12 games now and I didn't see one team better than us in these 12 games and we played against Liverpool and Tottenham
Maybe Sunday but we will see.
"(Man City) will take this game very seriously
They know that we are strong.
"It's one of the qualities is the organisation in the team and if you don't have that organisation it's not possible to concede only six goals in 12 games.
"They know, for sure, they have to be good to win against us and the challenge for us is to beat a good team in the Premier League.
"We have the supporters at our back and that will be important for Sunday."
While most onlookers have been taken aback by the manner in which Saints have started the campaign, the doubters will point to the level of opposition as a caveat to that success.
Tottenham and Liverpool are the only traditional powers they have come up against and represent Koeman's only defeats in all competitions.
That will change over the next nine games, though, with Sunday's match against City followed quickly by, amongst others, a double-header with Arsenal and home fixtures against Manchester United and Chelsea.
"I think if we would like to do better than last season, this period is important because we played nine games until January 1 and that's difficult, a difficult programme," Koeman said.
"Everybody knows there isn't an easy game in the Premier League and you have to be prepared every game.
"We know if we like to do something special this season then we have to win these kind of games that we have to play."
Key to a positive result on Sunday will be shackling in-form City striker Sergio Aguero.
Fresh from single-handedly helping City to a 3-2 comeback win against Bayern Munich, the 26-year-old is sure to give the Saints back line their biggest test yet.
"I think that the best player to compare to Aguero is Romario," former Barcelona defender Koeman said.
"I played with him but sometimes you think he is sleeping and in one split second he is scoring
That's a big quality because that means a lot of concentration for the defender.
"He's always fast, he's always on the shoulder of the last defender and that's his best quality."
One of those to play alongside Romario and Koeman in that impressive Barca outfit of the 1990s will be sat in the stands on Sunday.
Txiki Begiristain was an important member of the Blaugrana for many years and is City's director of football after holding that role at the Nou Camp for a number of years.
"I have good contact with him still," Koeman said
"We had a great time together in Barcelona.
"I saw him two weeks ago at a League Managers Association meeting about the work of Manuel Pellegrini and we had time to speak to each other about the past and about the future.
"We will see each other this Sunday and we look forward because these kind of games for us are the best to play."
Source : PA
Source: PA