It was Hart's error which led to the second, and ultimately telling, Saints goal, though any of a number of other players could have been called out for below-par performances.
Mancini told reporters (as per Sky Sports): "Joe has made too many mistakes. He has not been good enough. It was a really bad mistake from Joe. It was not good enough. It's not strange because he's done a few mistakes this year."
There is no particular suggestion that Hart's place is under immediate threat, but clearly the City manager has had enough of trying to cover for players who he feels are at least partially to blame for the team's likely failure to retain the Premier League title.
Mancini alludes to Hart's up-and-down form of this season, of which the fumble against Southampton was the latest chapter; here are five of his worst gaffes for club and country.