Back trouble proving a headache for Ferguson

John O'Shea's injury has robbed the United manager of options in defence – an area where the side is already struggling, writes Tim Rich

"You are all missing the point," Sir Alex Ferguson said with the exasperated air of a schoolmaster faced with some especially slow children. He was being questioned about what seemed like Manchester United's dangerously thin striking resources. "The biggest problem for us is getting our defenders back."

John O'Shea will not be back; not this season anyway. Like a lot of serious injuries – Gary Neville's ankle that kept him out for a year is a case in point – the Irishman's thigh strain did not initially seem that bad. He picked up a "dead leg" with the Republic or Ireland in the World Cup qualifier in Paris. Later, it was reported that there had been blood clotting in the thigh and yesterday Ferguson revealed O'Shea would be unlikely to feature again this season.

As his manager suggested, with O'Shea's departure United have lost a right-back, a centre-half and a left-back in one. If their challenge for the Premier League and European Cup is to extend into May, as it has done for each of the last three campaigns, O'Shea's role would have been an extensive one.

Twelve months ago, Manchester United's defence was functioning as smoothly as any back-four in world football. Between 8 November, when they went down 2-1 at Arsenal, and 21 February, when a jaw-dropping free-kick from Cristiano Ronaldo sealed victory over Blackburn by the same scoreline, they did not concede a goal. They had left the Emirates in fourth place, eight points behind Chelsea. By the time Blackburn were beaten, 14 clean sheets later, United were top, seven points clear of Liverpool.

And yet the greatest number of consecutive clean sheets kept by Manchester United this season is two and when last month they left Craven Cottage having been torn apart by Fulham, Ferguson complained that he had "a youngster and two midfield players in the back line".

Nemanja Vidic and Rio Ferdinand, the great cornerstones of United's defensive success last season, have played together just four times in the league and only once in a controlled 2-0 win at Stoke that Ferguson rates as one of the best displays of the season did they keep a clean sheet.

Two of the others contained moments cited as evidence for the decline of Ferdinand. One was the way he dawdled , appearing lost in thought as he sometimes did in the seasons following his arrival from Leeds, in the Manchester derby. Craig Bellamy pounced like a feral cat with fatal results. A month later, at Anfield, Fernando Torres muscled past him to score with an ease that would have been unthinkable six months before. A back injury that has troubled him before returned and Ferdinand has not played since. He is 31 now and, although Ferguson has made optimistic noises about his return, he will have to be nursed through until the end of the season.

Vidic is three years younger, but the man who was both the fans' and his team-mates' player of the year at Old Trafford has seemed curiously unsettled. His wife, Ana, has reportedly tired of Mancunian winters and both Barcelona and Real Madrid are ready with offers of warmer temperatures and fatter salary cheques.

When he withdrew with a pinched nerve just before United's defeat by Leeds, Ferguson showed little sympathy. And it was in this game and the subsequent home fixture against Burnley that the full decline of Neville's game was exposed. The finger pointed at Carlos Tevez at Eastlands was proof he will not be going quietly.

By no means is Ferguson's locker entirely bare. The Da Silva twins could be mightily effective in a year or two. Patrice Evra remains a commanding left-back and in the princely Jonny Evans United possess a centre-half who at 22 might surpass Charlie Hurley or David O'Leary as the finest defender produced by Ireland, north or south.

Nevertheless, there was a wistfulness in Ferguson's recent remark that if he could have found a high-quality defender who would have accepted a three-month contract, he would have signed him. But Sol Campbell returned to Arsenal and, however United are analysed, their manager is surely right. It's the defence, stupid.

Tao of Sir Alex: 'Obey me – or you're dead!'

Sir Alex Ferguson has warned his players they are "dead" if they step out of line and question his authority. The Manchester United manager also claimed he was the "most important" person at the club as he outlined his style of leadership in a speech to students in Dublin this week.

Ferguson, 68, admitted keeping control of today's highly paid players is one of the hardest tasks for the modern manager. He said: "Control is very, very important because if I lost control of these multimillionaires in my dressing room, I'm dead. So if anyone steps out of my control, they're dead."

Ferguson also underlined his own importance to United, having won the league 11 times at the club. "The most important person in Manchester United is the manager," he said. Few would dispute Ferguson's assessment, but it does demonstrate how much power he wields at the club and comes after the club's owners the Glazer family admitted last week: "Any successor to our manager may not be as successful as he has been."