The Manchester United monster is starting to look a little less fearsome – just as the club's fixture list becomes more daunting. The frailties that have emerged over recent weeks could be exposed more starkly over the month ahead. With a trip to Liverpool up next, followed, in the Premier League, by the small matter of the derby against those neighbours who are getting ever closer, Sir Alex Ferguson has plenty to ponder during the international break.

A few weeks ago it seemed that only City could disrupt the champions' progress to a 20th title. Then Chelsea pitched up at Old Trafford and, although they lost, pierced United's defence often enough to suggest that their transformation under André Villas-Boas is gaining momentum and United are far from the finished article. That point was then stressed by more modest opponents, as Stoke City and Basel, using very different methods, found abundant holes in United's defence.

The newly promoted Norwich City did too even though their primary mission was to contain the hosts – if they had finished better they would have left Old Trafford with far more than plaudits for an otherwise accomplished performance.

Luis Suárez and Manchester City's fleet of forwards are unlikely to be as wasteful as Elliot Bennett and Steve Morison were when allowed to run through free on goal only to dither, or when Anthony Pilkington had only the goalkeeper Anders Lindegaard to beat but shot wide; and on another day the Pilkington shot in the 76th minute that deflected off Anderson would have rebounded into the net rather than against the post. This was a most uncomfortable victory for United.

Injuries have undoubtedly contributed to United's disarray at the back. They have had to field six different central defensive partnerships this season and there have also been constant changes at right-back, while on the left Patrice Evra's mixed displays have added to the instability.

United experienced a similar turnover of defenders in the middle of their triumphant 2009 title campaign yet still managed to rack up a record 14 clean sheets in a row, but there are important differences between then and now. Two years ago Edwin van der Sar was on hand to help compose and organise the defence. Now United have no such experience in goal; David De Gea has been preoccupied with his own adaptation to the Premier League and, it seems, English life – he was consigned to the bench for the visit of Norwich the day after he was the subject of headlines involving larceny of a supermarket doughnut; his replacement, Lindegaard, is another Premier League newcomer and although his handling was good, his kicking was less so.

Another difference from two years ago is that back then the one player who did feature regularly was Nemanja Vidic, who was in imperious form. This term only Phil Jones has been ever-available and although the 19-year-old has impressed with his resilience and his breaks forward, there have also been times when he would have benefited from a partner who is experienced and fit – Rio Ferdinand is still looking rusty following his long lay‑offs.

Vidic has been absent since suffering a calf strain in the second game of the season and Ferguson is not sure when the Serb will be available again. Two years ago Ferguson could call on Gary Neville, John O'Shea and Wes Brown; this season he has had to depend on Jonny Evans – who endured another torrid game against Norwich – Antonio Valencia, whose deployment at right-back attests to Ferguson's admirable sense of adventure but also gives opponents another inexperienced defender to target.

Chris Smalling is likely to be fit in time for the Liverpool match and it will be interesting to see whether Ferguson deploys him at right-back, where he has excelled this season, or alongside Jones in a seventh central defensive duo of the season. The manager could also reassign Jones to right-back, where he began the season.

Jones, for his part, professes to be unconcerned by the chopping and changing. "It has been difficult to have a back four that we've used regularly all season but all the lads are capable of switching around the back four," he said. "The sign of champions is that you can play in any position and still prove that you're good enough."

There have been two constants amid all the changes. One has been Anderson, the midfielder who has grown in significance this season and emphasised that by heading the breakthrough goal against stubborn Norwich, before Danny Welbeck tapped in a late second. The other has been results: they have yet to lose. That could change in October.