Manchester United's Carling Cup defeat by Coventry City - taken in isolation - hardly drives a stake through the heart of Old Trafford.

But scratch beneath the surface of United's humiliation against the Championship side and there are worrying signs for boss Sir Alex Ferguson.

True, United's boss chose to re-shuffle his side, changing his team entirely after Sunday's win against Chelsea.

Any shock result must be firmly placed in that context.

And of course a full-strength United side will have every chance of retaining their Premier League title.

But what of United's shadow squad? Those who could be called into action if Ferguson's side are struck down by weight of injuries?

And how good are they in comparison to those on the under-card at Chelsea, Liverpool and Arsenal?

On the basis of impressive victories for those three and a miserable defeat for United, not very.

And it's a fair bet this will be troubling Ferguson as much as the loss to Iain Dowie's side.

Ferguson may not place the Carling Cup high on his list of priorities, but do not tell him it doesn't matter - and the old fury was blazing in his eyes at the final whistle.

And it will only get worse when he scans those waiting in the shadows at The Emirates, Stamford Bridge and Anfield.

Arsenal's youngsters are maturing before our very eyes and saw off a strong Newcastle United with serious intentions of winning the Carling Cup under Sam Allardyce.

Chelsea had too much quality all over the park for Hull City and Liverpool have their strongest squad in recent memory.

Rafa Benitez has a four-pronged attacking threat, led by the brilliant Fernando Torres, while in central midfield they can choose from Steven Gerrard, Xabi Alonso when fit, Momo Sissoko and Javier Mascherano.

United have a strong main body of a squad, but it is questionable whether they have the depth to match their title rivals.

Ferguson will have wanted his youngsters to make a statement against Coventry - and perhaps they did.

Phil Bardsley, Jonny Evans, Gerard Pique, Chris Eagles, Dong
Fangzhou and even expensive summer purchase Anderson would not trouble the scorers at Arsenal, Liverpool and Chelsea.

This will be a telling factor for Ferguson when he picks through the wreckage of Coventry's raid on Old Trafford.

Ferguson is the master at manipulating his resources and no-one would suggest for one second that the setback against Coventry will have a huge impact on their title chances.

But he will have left Old Trafford after the loss with maybe a touch of envy at the strength and variety of squad available to his title rivals.

Do you think United have the weakest overall squad among the big four?