Manchester United manager Sir Alex Ferguson.

Dublin – Sir Alex Ferguson has admitted that his defence is more vulnerable than at any time in his 26 years at the helm of Manchester United and declared that if the side defend in Sunday's Manchester derby as they did at Reading last weekend “then God knows what's going to happen to us”.

Ferguson said on Tuesday he would play Wayne Rooney against Cluj in Wednesday’s Champions League dead rubber at Old Trafford because his metabolism requires three games for him to hit match fitness. But with the manager claiming that his captain, Nemanja Vidic, is fit neither to face the Romanians tonight nor City on Sunday – which may have been one of his decoy statements – it is the defence that leaked three goals at Reading which concerns him.

Asked about next weekend's derby at the Etihad, he said: “Last weekend? Ha! Cartoon cavalcade. I think we're getting punished for every ball that goes in the box at the moment but, saying that, we should still be defending far better. Reading produced some fantastic balls into the box, but we do and should be expecting better. It's a worry, it's a concern.”

United have conceded 32 goals in all competitions and only three times before in Ferguson's reign have they reached Christmas having let in more: 1998-99 and 2001-02, when they shipped 41 on both occasions, and 1999-2000, when they let in 35. That one of those campaigns should have been United's Treble-winning season shows that the goals conceded column does not tell everything.

Ferguson was upbeat and more responsive than at any such Champions League gathering this season. But when it was put to him that this season might pose the worst defensive problems he has encountered, he did not disagree. “Well, you see the goals against: we've lost 32 goals this season,” he said. “I can't remember us losing so many goals before Christmas. I must say that. So it is a concern, there's no question about that. When you look at the side City could put out, with [Mario] Balotelli, [Edin] Dzeko … all really tall players, it's going to be a really big challenge for us on Sunday.”

Sunday is undoubtedly playing on his mind, though. Last October's 6-1 thrashing at Old Trafford still weighs heavily, one of two Premier League defeats to City in 2011-12, and the manager hints in tonight's programme notes that he will abandon any swagger and go for safety, at a time when his defence cannot be trusted.

“We need to come out unscathed,” Ferguson writes. “With so much at stake I wonder if there will be a bit of caution from both clubs [on Sunday]. Roberto Mancini and I will both have to think deeply about our selection and tactics and there may well be an element of playing safe. But don't count on it because it's a derby match when anything can happen and often does. The passion of the fans can easily spill over so that tactics go out of the window as emotions take over. We could be in for a masterclass of attacking football from both teams – in which case, hold on to your hats and enjoy.”

Ferguson did not reject the well-argued case offered by Gary Neville, on Sky TV on Monday, that United's goalkeepers must be more commanding in coming for crosses into the box, which are proving to be United's undoing. “Well, they are getting free headers and that's the issue we've got to face - and we've got to arrest that,” he said. – The Independent