Manchester City's 100% winning start to the Premier League season came to an abrupt end against Aston Villa - leaving Enzo Maresca with glaring defensive issues to resolve.
Nicolas Jackson's double in a 2-1 home win for Villa means City have now managed only one clean sheet in their past six games and the form of goalkeeper Gianluigi Donnarumma and defender Marc Guehi is of concern.
Since the start of last season, only eight players have made more errors leading to Premier League goals than Donnarumma and Guehi with three each.
City's most recent shutout came in their impressive victory at Liverpool two weeks ago but, around that game, City have conceded against teams ranging from Champions League holders Paris St-Germain to top-flight strugglers Sunderland.
Maresca was forced to try different combinations at the back against Villa, but as the legendary Sir Alex Ferguson once said, "attack wins you games, defence wins you titles".
Though still early days in the campaign and guaranteed to be in the top two come the end of the weekend, City need to shore up their backline if they are to stay there.
Asked if the two goals were avoidable, Maresca told TNT Sports: "Absolutely, both.
"Sometimes you have to be a bit more lucky because we only conceded those two chances, but we created a lot of chances.
"We played most of the game in their box but you have to be clinical and avoid mistakes like the two goals we conceded."
City were undone by goals in each half from the brilliant Jackson at a raucous Villa Park to end their streak of seven straight wins at the start of this campaign.
Maresca's team started the season by conceding eight goals in their first nine games but that has seen a dramatic downturn, with nine goals hitting the back of their net in the past four matches.
The signing of Donnarumma last summer was heralded as the acquisition of one of the best goalkeepers in the world to replace Ederson.
The Brazilian changed the face of English football with his footwork, spraying short and long passes across the pitch, and there are few keepers who can come close to matching that.
But Italy captain Donnarumma struggles with the ball at his feet and often puts his team-mates in trouble with passes out from the back.
On Saturday, City captain Ruben Dias gesticulated to his team-mate to send the ball up the pitch after the Portuguese failed to control a difficult pass.
Maresca decided to sell James Trafford to Leeds for £45m in the summer, and the England international has been impressive at Elland Road.
Trafford has made more passes than Donnarumma this season, has a slightly better rate at preventing goals and has come out to sweep the ball 11 times compared with Donnarumma's one.
Though Donnarumma could do little about Jackson's thumping finish for the opening goal, he should have done better for the second as a low shot seemed to squirm under him.
Former City defender Steph Houghton said on BBC Radio 5 Live: "I think Villa really utilised keeping the ball and being a real threat to Manchester City. Donnarumma, especially with the second goal, could probably have done a little bit better.
"But I don't want to take anything away from Villa. I think they performed really, really well and the gameplan was spot on, just to nullify City as much as possible."
Centre-back Guehi's £20m arrival from Crystal Palace in January was considered a bargain.
But it was his loose pass at Everton late last season which cost City dearly in the title race, drawing that game and losing the impetus to eventual champions Arsenal.
This season has been a struggle for the England defender, culpable for both goals scored by Ipswich last weekend and being picked off by John McGinn deep in his own half for Villa's first goal.
Later in the half, he hauled down Jackson on the edge of the box before being shown a yellow card, and was put out of his misery by being taken off at half-time.
City sources had said they were targeting signing a right-back in the summer before pivoting elsewhere, saying Abdukodir Khusanov can provide back-up should Matheus Nunes not be available.
Khusanov started out wide but struggled before being moved inside in place of Guehi, while Rico Lewis was brought to fill the gap, with Maresca acknowledging afterwards he needs to "find that balance" in defence.
Villa supporters provided constant reminders to the travelling fans of their side being "cheats", and the vitriol faced following the guilty verdict of the financial charges may be beginning to weigh on the players' minds.
Asked by BBC Sport if Guehi's form is a worry, Maresca said: "No, I'm not worried about Marc at all.
"I know he is playing every minute since the season started with us and the national team, so it's not easy but the change was also because he was on a yellow card.
"The way we want to be aggressive sometimes, to have one yellow card can be dangerous but I am not worried at all."
Maresca, though, should be concerned by coming up against Fabian Hurzeler's free-scoring Brighton twice next week - in the Carabao Cup on Wednesday, and Premier League on Saturday.
The Italian needs to fix the defensive leaks before they become something more serious.