Joe Hart kept 17 clean sheets in the Premier League last season as he helped City win the title, this year five matches in he is yet to keep one.
Add to that the three goals conceded in the Bernabeu, and the start to City's season has been far from ideal. Three draws means City currently sit four points behind league leaders Chelsea, who have won four and drawn once.
At the end of last season it was the clean sheets kept at home to rivals United and away to Newcastle which played a crucial role in their title, but that form has not carried over, with Southampton setting the tone when they scored two at the Etihad on the season opener.
Yesterday they failed to hang on for a 1-0 victory over Arsenal, conceding a late equaliser to tie the game a 1-1 and drop two points. There is little doubt that Roberto Mancini, a man who hails from Italy and played in the defensively aware Serie A, will not tolerate his side's generosity for much longer.
So what is going wrong at the back? City's defence was so good last season, why the drop off in standard? There will always be questions to be asked about team selection elsewhere in the pitch, but the defensive problems are proving a constant irritation, and without wishing to overplay them, they deserve a bit of focus.
There are a number of reasons for the problems, starting with Roberto Mancini himself. His team selection and centre-back rotation is a causal issue of the club's lack of solidity.
If you take City' three biggest games this season, against Liverpool, Real Madrid, and Arsenal, Vincent Kompany has had three different defensive partners. Kolo Toure against Liverpool, Nastasic at Madrid, and Lescott at home to Arsenal.
This can't help, and is the sort of over-rotation that Manchester United were guilty of at the start of 2011-12, which cost them early season points. Mancini needs to pick a partner for Kompany, and stick with him.
His partnership with Lescott was largely successful last season, and while Mancini will have been annoyed to have conceded twice against Southampton, dropping him for Toure in the following game wasn't helpful to anyone.
City are this season, like last, alternating their choice of full-backs, with Kolarov and Clichy on the left, and Zabaleta and Maicon on the right, which will be further complicated when Micah Richards returns.
Bearing this in mind, rotating the centre-backs is only going to add to a lack of stability, and a regular central partnership can only benefit whoever comes in at full-back, and will be helpful to Joe Hart in goal too.
Unfortunately a League Cup tie this week will likely lead to more rotation, with Kompany expected to be rested himself, and Nastasic a probable to come in.
There are two other contributing factors to the leaky defence, another being Mancini's pre-season experiment. He toyed with playing three at the back in pre-season, and the Community Shield, which did not work, despite the club's 3-2 win.
This meant City had gone into the season playing one season, only for Mancini to abandon it when the Premier League kicked off, and while reverting to four at the back should have been routine, it has been problematic still.
The other key underlying problem, is the inconsistent midfield defensive shield, with Gareth Barry and Nigel de Jong so often implemented so effectively. Barry began the season injured, and De Jong was sold to AC Milan, with some arguing the latter has weakened the squad.
Jack Rodwell was certainly tentative in his early performances, giving away possession which led to a Southampton goal on his league debut for the club, but the signing of Javi Garcia has strengthened that department along with Barry's return from injury.
However the area, like defence, is one which has seen a fair share of rotation, so it is hard to argue the inconsistency has not been a factor. City will get it all corrected eventually, but it will need to be sooner rather than later. A clean sheet away to Fulham next weekend would be most welcome, but the way things are going, don't bet on it.
What do you believe has been the biggest factor in City's failure to keep a clean sheet?
image: © wonkerq