There were times at the Etihad Stadium on Wednesday night when it was Joe Hart against Borussia Dortmund. The German champions had 22 shots in total, as they opened Manchester City up time and again, and it is no exaggeration to say that eight of those attempts produced world-class saves. "It was almost like being down at the park and being peppered by your mates," said Joe Hart.

It was a breathtaking display of goalkeeping from a man who had been told by Roberto Mancini two weeks earlier to "stay in goal and make saves" after he dared to criticise his team-mates for allowing a 2-1 lead to slip against Real Madrid. This time the City manager was one of many patting the England international on the back after a performance that prompted Wayne Rooney to declare that the 25-year-old from across the city deserves to be called the "best goalkeeper in the world".

Rooney knows how to start a good pub debate. Iker Casillas, Gianluigi Buffon and possibly Manuel Neuer would have something to say about the merits of Hart being feted as the No1 in his position. It is not, however, something that Mancini will waste much time pondering. The more pertinent question for the City manager to find an answer to over the coming weeks is why does the goalkeeper of the Premier League champions keep having to be so good?

Even allowing for the calibre of City's opponents in their opening two Champions League fixtures, it must be sobering for Mancini and his staff to look at Opta's facts and figures and see that Real Madrid and Dortmund had a combined 57 attempts on goal against them. City, to put it another way, have been in danger of conceding a goal every 190 seconds in the Champions League. Hart, in those two matches, has had to make 18 saves, which is four more than any other goalkeeper in the group stage of the competition.

Although Hart has not been overworked to that extent domestically, there are legitimate concerns about the ease with which the defence in front of him has been breached. City have yet to keep a clean sheet in any competition this season – they had four at this stage last year – and they have let in 16 goals in nine matches. Had Hart not been in such incredible form against Dortmund, they could easily be looking at a record of conceding an average of two goals per game this season.

So where is it all going wrong? The first thing to say is that the central defensive partnership of Vincent Kompany and Joleon Lescott, which served City so well last season, has been broken up. Lescott, who seems to have paid a heavy price for a recent mistake against Arsenal, was left out against Dortmund, just as he was against Fulham at the weekend and Real Madrid and Liverpool before that, with Mancini preferring to go with the 19-year-old Serbian Matija Nastasic. It is hard to say whether Kompany's form has been affected as a result, but there is little doubt that the City captain has not looked as assured as he did last season.

It is too easy, though, to lay all the blame at the door of the defence. City's problems without the ball run deeper than their centre-half pairing, as Mancini alluded to after the Dortmund match. Mancini's side made 293 more passes than Dortmund, yet the Bundesliga club, through swift counterattacks that exposed the lack of protection in front of City's defence, registered 22 attempts on goal compared with 12. City, in other words, were cut to ribbons whenever they lost possession. "We can't lie, we weren't great," Hart said. "We were a bit open, but we did create chances."

Managers and coaches often talk about "the moment of transition" as being crucial, namely when possession changes hands and how players react. It was one of José Mourinho's favourite themes on the training ground at Chelsea and appeared to be an area of weakness that Mancini touched on in the wake of the 1-1 draw with Dortmund when he talked about the need to defend with "all [of the] players".

There should, in theory, have been a defensive shield provided by the holding midfielders, Yaya Toure, Javi García and Jack Rodwell, who replaced the Spaniard in the first half and was at fault for Dortmund's goal. In the end, though, the only thing that came between Dortmund and a convincing victory was a goalkeeper playing out of his skin. "Was it my best performance for City? I don't know about that," Hart said. "All I know is it got us a point and keeps us in the running for this group."