It was not just that they won 1-0, it was the manner of the win, overcoming an injury crisis and defending with implausible resolution for 90 minutes before striking through Ji Dong-won in injury-time.
Almost a year on and the landscape could hardly be more different for Sunderland, even though they are in the same 15th position in the league, as they were before that game.
A year ago, there was optimism and a sense of something building on Wearside; talk, even, of a challenge for Europa League qualification this season. Now, the only aim is avoiding relegation.
Wins over Reading and Southampton have eased concerns to an extent, but a Christmas programme of City and Tottenham Hotspur at home followed by a trip to Liverpool means that the Black Cats could easily be in the bottom three in a week's time.
That is why the win over Southampton was so important in providing at least a little breathing space. "We approach them with a bit of confidence," said Steven Fletcher, who scored the only goal at St Mary's.
"We know we've got a hard Christmas period coming up but we've built a bit of confidence and we'll take that into the next game. The boys are all very tight. We all get on very well. It's a good bunch of lads."
Team spirit is all very well, but Sunderland expected a little more than that. Fans have remained loyal, though - for all the ill-informed stories about O'Neill being on the point of resigning, or the board or supporters losing patience, there has been remarkably little dissent around the Stadium of Light - and there is a strange sense of bewilderment about the lack of form.
With almost half the season gone, Sunderland are four points better off than they were in equivalent matches last season but they must be concerned at the number of points that have been squandered by conceding late goals (to Liverpool, West Ham, Everton) or soft early ones (they played well against West Bromwich Albion and Norwich City, but only after going 2-0 down).
Last year's City game, O'Neill said, is a source of inspiration, a reminder of both what is possible and the potential at the club.
"It was a terrific win," he said. "There was a lot of character shown that day. I know these are over used words in the game, but that was very important for us because Manchester City at any given stage in that match or in any game, can rip you apart."
As footage of a fan kissing Ji became a sensation in South Korea, that roar was seen by some as the waking of a beast, a reminder that Sunderland is a club capable of drawing 48,000 supporters.
O'Neill's job is to make sure that those fans can look forward to a happy new year this time around.
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