European reality checks
Rewind to the beginning of this week, and both the Saints and City would have been licking their lips at the mouth-watering prospect of cementing their status on the European stage.
For the Premier League leaders, it would be a Champions League trip to Spain and the eagerly-anticipated return of manager Pep Guardiola to his old stomping ground the Camp Nou, where the legendary Barcelona side that he created where waiting for them last Wednesday. The stakes where immensely high. This was the chance Man City had been waiting for; the opportunity to prove that they were serious contenders to sit at the very summit of European club football after years of steady progression.
If City could do it, if they could re-discover that form that had blown away their cross-town rivals Manchester United just a few weeks ago, then they would not only overtake Luis Enrique's side as the leaders of their Champions League group, but send out a serious message to the rest of Europe.
However, what followed was quite simply overwhelming evidence that City are still not at that very top level. Barcelona, led by hat-trick-hero Lionel Messi, blew Guardiola's men away, winning 4-0 and emphatically emphasising both their own and their visitor's position in the European hierarchy.
Southampton meanwhile, travelled to Milan for their momentous Europa League clash with another traditional European giant, Inter, just 24 hours later. Like City, they understood a win would let the continent know that they were forced to be reckoned with this season, as they looked to put breathing space between themselves and the chasing pack in Group K.
It could be argued that a trip to the San Siro was the greatest day in little Southampton's recent history, regardless of the result, but with the Nerazzurri reeling from issues on and off the pitch, this truly was an opportunity not to be missed.
As a result, it wasn't too big a surprise when Southampton dominated the contest from start to finish. They had all the ball, all the chances but, a familiar problem resurfaced. The Saints simply could not find a way to find the back of the net. On the flip side, Inter and Antonia Candreva clinically converted their only real sight of goal and Claude Puel's men, somehow, went down 1-0.
Now, it is straight back to business for both City and Southampton, with the problems that surfaced in the week still very fresh in the memory.
For City, the shocking and harrowing manner of defeat in Spain will have put their confidence on the floor. Being the home side and table-toppers, they will start Sunday's encounter the clear favourites, but will be feeling all the pressure to get back to winning ways after the Catalonian Catastrophe.
The Saints on the other hand experienced the opposite kind of loss in Europe. Nevertheless, the defeat will serve as a serious wake-up call on the South Coast; they have a serious problem when it comes to finishing their chances. Despite the Inter game being their first loss in eight matches, Southampton fired blanks at Leicester City, Hapoel Be-er Sheva and of course the San Siro in this period. With more ruthlessness, Southampton could have won all three of these games comfortably.
What's more, Southampton have created the second-most chances in the Premier League this season (111). However, because of their atrocious shots-to-goal ratio (14.6 shots per goal), the most in the country's top division, they sit a modest eight in the table.
So, it is clear that both sides have discovered major problems with their own game in the past few days. Consequently, it looks likely that the victorious team when Man City and Southampton clash on Sunday will be who is quickest to put their European nightmares behind them, and put right their problems from the week. Whoever possesses the superior mental toughness will surely come out the winners in this fascinating contest.
City will be without a recognised right-back on Sunday, with both Bacary Sagna and Pablo Zabaleta sidelined. Skipper Vincent Kompany is also a doubt.
For the visitors, a hamstring injury to Shane Long on Thursday mean he will not feature, while Ryan Bertrand, Matt Targett and Cedric Soares all face a battle to be fit for the game. Summer signing Sofiane Boufal is expected to start on the bench again as he continues to regain match fitness, after making his long-awaited debut for Southampton against Inter.
There's history between them
It was a two home victories when the clubs met in the league last season, with City winning 3-1 at the Etihad, before Southampton got revenge with a 4-2 win at St Mary's.
Veteran Saints goalkeeper Stuart Taylor is the only player in either squad to have been both a Man City and Southampton player, although he is yet to play his first game for his current club.
In 84 league matches between the teams, Southampton have won 31, while Man City have come out on top 29 times.