Where's Jonny? Evans' fall from grace gives Ferguson defensive derby worry

The official word is that Jonny Evans just happened to be the first defender in the manager's line of fire when Manchester United looked so vulnerable to Milan's early blitz in San Siro in the Champions League a year ago next week. "Fucking wake up," Sir Alex Ferguson screamed at him as he was restrained by assistant Mike Phelan after Paul Scholes had cancelled out Ronaldinho's opening goal. Ferguson later described this as "a gentle bit of advice" for Evans in a partisan atmosphere in which he struggled to be heard.

But the benefit of 12 months' hindsight tells us that Evans' problems were a precursor of a more fundamental struggle for form. They contribute to United heading into Saturday's derby against City at Old Trafford with a substantial central defensive headache.

The slim hopes which might have existed early on Tuesday of Rio Ferdinand being fit for Saturday appear to have evaporated by the afternoon, which means that Ferguson is now looking for someone else to partner Nemanja Vidic. A year ago it would have been Evans, whose departure from the Northern Ireland squad with an ankle problem on Tuesday, just as United's training session came to an end, could be interpreted as the result of a discreet phone call from Manchester. Now, the 24-year-old looks a less likely candidate to replace Ferdinand and, with Wes Brown having almost vanished from Ferguson's radar, 21-year-old Chris Smalling, who has only three Premier League starts, looks like the one who might be given the responsibility.

There will be no sentimentality for Evans, of course: Ferguson has shown his most ruthless streak with central defenders and never more so than when Gary Pallister found himself out of Old Trafford within weeks of the end of the 1997-98 season after his own rapid decline contributed to United letting slip a 12-point lead in the title race to Arsenal. But Ferguson's underlying belief in Evans will take some denting: he was an integral part of the United side which went 14 games without conceding two seasons ago and they let in seven goals in the 13 games he played in the first half of that campaign.

Back then he was largely unknown, though, surprising Chelsea with a defiant Premier League debut for United at Stamford Bridge in September 2008. Now, he is subjected to the same scrutiny as the rest and it is his strength and positional sense when high balls come into the box which have been found most wanting. Everton's Tim Cahill had the better of him at Goodison Park; Carlton Cole inflicted his most miserable night yet in the 4-0 Carling Cup defeat to West Ham at Upton Park. Evans is being bullied by strikers. Ferguson has not always been enamoured of Alan Hansen's opinions about his players this season, though the pundit's view that teams target Evans, rather than the more obdurate Vidic – and by doing so affect the Serb's own composure – seems a valid one.

Brown's inclusion against City would represent just as much of a gamble, because he has started only two Premier League games this season, giving away a penalty in the 2-2 draw at Villa Park in one. His departure to the sidelines is less fathomable, the suspicion being that the row he is believed to have had with Ferguson during United's pre-season tour of America – possibly about the manager's unwillingness to sanction a night out – has contributed.

None of the potential partners has grown accustomed to each other, though. This season's longest unchanged central duo is the four and a half games Ferdinand and Vidic had in December. Statistically, Vidic and Smalling are a more resolute unit (one goal conceded in every 163 minutes of football) than Vidic and Evans (one in 70 – though the West Ham game skews things.) Smalling's form has not been peerless. He was caught out of position in allowing Tuncay Sanli to cross for Stoke City's goal at Old Trafford last month and the suspicion is that he needs at least another year to develop as the heir to Ferdinand. The 4-3 scoreline from last season's Old Trafford derby was one which owed much to Ferdinand's nonchalance but he is certainly the best communicator in United's defence. Ferguson still misses him more than any other defender.

The fact United have conceded a mere eight goals at Old Trafford all season provides an essential piece of perspective, but the threat possessed by 6ft 4in Edin Dzeko will be at the front of the City manager Roberto Mancini's mind. The Bosnian will probably be on the bench, though he got the better of a Vidic/Ferdinand partnership for Wolfsburg in the Champions League at Old Trafford 18 months ago and, having already observed that Manchester is full of City fans, he will not doubt his own ability to deliver a menace to go with his mischief.