Sir Alex Ferguson described his as a "disaster"; Roberto Mancini looked bemused as he muttered: "It's incredible," while Andre Villas Boas could only fall back on the bluntest of realism: "Our squad is not good enough to win the league." The fixtures on the last day of 2011 and the first of 2012 brought an air of consternation to the top of the Barclay's Premier League, and with it a welcome sense of uncertainty.

Since the two Manchester clubs detached themselves at the top of the table early in the campaign, the intrigue seemed to lie in how their relationship would be shaped or distorted by the added consequence of the championship being the prize of their rivalry.

They can still find reassurance in the six-point gap that separates the pair of them from the rest of the league, but the idea that the title would be awarded at the end of a procession has been temporarily abandoned.

United's defeat by Blackburn Rovers on Saturday infuriated Ferguson, not least because it exposed the doubt that resides not only in the far reaches of his squad – Antonio Valencia at right-back, Michael Carrick at centre-back, a bench that included only one recognised first-team player in Anderson – but also the continuing lapses of judgment by Wayne Rooney. As Old Trafford agitated at a 3-2 loss, Rooney was confined to the stand after socialising on Boxing Day and apparently being lethargic at training the following day.

City's 1-0 defeat at Sunderland owed more to the doggedness that tends to be the first expression of Martin O'Neill's work at a new club, but also the kind of random misfortune that causes a manager to feel inconsequential. Even with 19 shots on goal, and twice hitting the woodwork, City were left unrewarded, then succumbed to a last-minute counterattack in which Ji Dong-won scored from an offside position.

There are occasions when results have to be borne, and there is greater cause of concern at Old Trafford since Ferguson's squad looks thin and the team has misplaced some of its customary ruthlessness. The effect of enduring a run of games across the festive period cannot be discounted, and results are often unexpected during a spell that enforces mental as well as physical fatigue. Yet Aston Villa's victory at Stamford Bridge on Saturday felt like a confirmation of Chelsea's diminished nerve.

Villas-Boas is in the midst of a period of upheaval, and Didier Drogba's opening goal was followed by the Ivorian saluting each corner of the ground in an overblown gesture that was interpreted as an act of farewell. Others will follow – and Frank Lampard was unusually slapdash as he conceded the ball for the visitors' third goal – but even lavish spending in January is unlikely to restore Chelsea's title challenge.

Instead, it is the verve of Tottenham Hotspur, even if they were held to a draw away to Swansea, and the steady resurgence of Arsenal that will gather into the most pressing threat to the two Manchester clubs. Such a scenario seemed absurd after the former lost their opening two league games, against United then City, to an 8-1 aggregate scoreline, while Arsenal were overwhelmed 8-2 at Old Trafford. A depth of competitiveness has returned to the Premier League and Spurs would move to within three points of the joint leaders if they win their game in hand against Everton.

With Liverpool and Newcastle United proving capable of matching the sides above them, as well as occasionally faltering against those below them, the top half of the table looks congested rather than split into different elites. There is also a cluster of teams at the bottom, with five sides directly involved in a scuffle to avoid relegation. The woes of Bolton seem deep set, but Owen Coyle will have scope to work in the transfer window once Gary Cahill completes his multi-million pound transfer, most likely to Stamford Bridge.

Steve Kean has become an icon of grim perseverance at Ewood Park, and drawing with Liverpool then defeating United might act as a small release of the tension and loathing forced upon him from the stands. Wigan, too, are unpredictable, and although the Premier League can still be accused of self-absorption, results during the festive period cast some disorder into the hierarchy that has been established this season.

There is suddenly hope for the bottom clubs to build upon, and faltering for those at the top to overcome. Most other European countries treat Christmas and New Year as a respite from the game, but the British tradition of cramming matches into the period is riveting as well as being quaint. The Premier League has become a more compelling prospect for the quirky results this spell has amassed.