Gary Neville has claimed Manchester United's defensive frailties can be explained away by their constant changes of personnel.

After last season's record-breaking feats, when they kept 14 successive Premier League clean sheets, United have been strangely vulnerable this term.

Tuesday's six-goal Champions League thriller with CSKA Moscow was the second time they have conceded three goals in a game, while their seven shut-outs in all competitions this term represents less than a 50 per cent success rate in keeping the opposition out.

Neville accepts there is an issue to address. But he is convinced once Sir Alex Ferguson is able to select a settled back-four again, normal service will be resumed.

"We are interchanging the defence too regularly at the moment because of injuries and it has caused us a problem," he said.

"We had to change our keeper for the opening two months of the season, then Rio Ferdinand and Nemanja Vidic have been injured. Before that it was Wes Brown.

"It means more changes than the manager would like.

"He wants a consistent back four, which is what we have had for two or three seasons.

"We definitely want to improve defensively and once we settle into our stride I am sure we will become better."

Neville did not do much to help the situation when he was sent off in the Carling Cup win at Barnsley last week.

It earned the United skipper a three-match domestic ban which rules him out of Sunday's eagerly-awaited showdown with Chelsea at Stamford Bridge.

John O'Shea is almost certain to step in, although Ferdinand is again set to be on the sidelines with a continuing calf problem, even if Nemanja Vidic has recovered from a similar injury.

As Patrice Evra will also return to the starting line-up in place of Fabio, whose defensive discipline let him down against CSKA in midweek, it means three-fifths of United's defence will have changed in the space of five days, just highlighting the point Neville is making.

Not that the veteran full-back, who has been tipped for an England recall for next week's friendly with Brazil in Qatar, believes there is any basis for the emergence of so many doom-merchants recently.

"Things are not too bad here," he reflected.

"We are through to the quarter-finals of the Carling Cup, we are two points off the top of the league - which is probably the best start we have had in years - and we are through to the knock-out phase of the Champions League with two matches to spare.

"It is always the same at this club. When you lose a game at Liverpool there is criticism afterwards.

"But we have beaten Arsenal at home, we have beaten City at home. We have had other good results. We are not concerned by anything that happens externally."

Ferguson has already identified Chelsea as the main threat to United's bid to land an unprecedented fourth consecutive league title.

Neville is not willing to make such a quick judgement. He is aware Arsenal will draw level with the Old Trafford outfit if they win their game in hand.

In addition, big-spending Manchester City are not that far behind, while he remains wary of old rivals Liverpool, despite their current turmoil.

"It is too early to say where the biggest threats are going to come from," he said.

"At the moment Chelsea are the team on top, so they are the ones we are all going for.

"But there are three or four up there who are capable of putting challenges together."

What Neville does accept is that results between the major contenders can have a significant effect, even if United landed a record-equalling 18th league championship last term despite winning just once against the established 'big four'.

"These matches are always key," he said.

"It is a big game between two big clubs. Both teams will want to win, but both also know it will be just as difficult as ever.

"No matter what happens though, there is still a long way to go."