Having splashed around 90 million pounds on bolstering their attack with Jesus Navas, Fernandinho, Alvaro Negredo and Stevan Jovetic, it was their lack of spending at the other end of the pitch that was exposed in Wales on Sunday.
They lost Kompany to a groin strain during last week's Premier League opener against Newcastle United and were already without Nastasic who is recovering from an ankle problem, forcing them to field a makeshift partnership in Cardiff.
Manager Manuel Pellegrini chose not to blame Joleon Lescott, the England defender who lost his place to Nastasic last season, or Javi Garcia, the midfielder who was drafted in to play alongside him, saying there was a collective responsibility.
"I haven't seen the goal yet but defending set pieces is the job of the whole team. It doesn't matter who is guilty but to concede two goals from corners is too many," the Chilean said on the club website.
City had gone ahead in the game with Edin Dzeko's 52nd minute goal before Aron Gunnarsson levelled in the 60th and last season's league runners-up were then undone by two Fraizer Campbell headers from corners in the space of eight minutes.
Negredo pulled one back for City in stoppage time after coming on as a substitute for a second successive game and again showing just the kind of sharpness the club had been looking for when they signed him last month, but it was too late.
"We lost because we didn't concentrate. We will work on this during the week," said Pellegrini, whose side had stormed to a 4-0 win in their match at home to Newcastle with a delightfully attack-minded display.
MEANEST DEFENCE
He had said before the game Kompany's absence was not a reason to bring in a centre back before the transfer window closes on September 2 but after last term's meanest defence leaked goals against Premier League newcomers, he may change his mind.
Kompany has been ruled out for around a month, making him a doubt for the first Manchester derby of the season when City host champions United on September 22, while Nastasic has only just returned to training.
With Micah Richards out with a hamstring problem and having seen Kolo Toure depart on a free transfer to Liverpool, City find themselves depleted at the back with the season only just underway.
They have the financial means and the time to do something about it, should they wish to, and having been given a target of five trophies in five seasons Pellegrini could still take their close-season spending over the 100 million pound mark.
City, who won the Premier League in 2012 and the FA Cup in 2011, have been linked with Real Madrid's Pepe and Atletico Madrid's Martin Demichelis in this transfer window.
Under manager Roberto Mancini last term, City let in 34 goals in 38 games and in the previous season they had an even better record, conceding just 29 on their way to the title.
Doubts over their back line had surfaced during the pre-season when they let in eight goals in their last three warm-up matches and it is certainly not a rate they will want to maintain.
(Editing by Toby Davis)
