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.
We start off with an international fixture and a poor misjudgement which wasn't helped by a two-man wall, which fell apart in an instant.
Hart was caught out by a relatively straight-forward free kick which curled past the wall and inside of his near post, despite his late attempts to keep it out.
Being beaten at the near post is always a big no-no for a top class goalkeeper, and this wasn't exactly a difficult attempt to spot coming.
No video, unfortunately, but Hart got himself in a tremendous mix-up with teammate Kolo Toure to allow Nikola Kalinic an easy tap-in against Blackburn Rovers in 2010.
Both defender and goalkeeper went for the ball, Hart called it and Toure left it—and then so did Hart, letting in the Rovers striker to finish with ease.
This mistake came not too long after Hart had established himself as England's first choice goalkeeper, but he's still making these errors now, it seems.
Being beaten at his near post is something of a bad habit for Joe Hart.
Here, former teammate Adam Johnson cuts infield from the right and strikes a low shot toward near post, which beats the keeper all ends up.
The effort is firmly hit, but it's terrible positioning and sluggish reactions from the goalkeeper which really allows the goal to creep in.
Bouncing ball, slippy conditions, goalkeeper under pressure, a cup replay at stake...it all adds up to one thing.
Hart simply couldn't handle a fairly innocuous cross from the left, spilling it into the path of attacker Andy King and then compounding his error by not making the block at the second attempt, letting the forward nip in to score.
This was a late equaliser and the game ended 2-2, forcing a replay.
On to the most recent occurrence to finish with; Hart should have taken a thoroughly routine catch at his near post after the attacker cut infield to shoot.
Slippery ball? Took his eye off the play? Worn-away gloves? Another attacker distracted him?
Whatever the reason, Hart completely failed to deal with a very basic catch and let the ball trickle through his legs for the oncoming attacker to stab home from close range.