Graphic: pass master Anderson

These should be the sweetest of times for Manchester United, back on top of the Barclays Premier League and looking forward to a Champions League semi-final against Arsenal, but Sir Alex Ferguson’s expression at the final whistle betrayed a nagging sense of unease.

Beating Liverpool to the title was supposed to be fun — and no doubt it will be when the time comes — but this, not for the first time, had the feel of a difficult slog that carries some worrying portents for the forthcoming battles on the European front.

No victory in the Premier League should be sniffed at — particularly not when Liverpool received so many plaudits for an error-strewn performance 24 hours earlier in what is likely to prove a costly 4-4 draw with Arsenal — but United are toiling and finding the going tougher than Ferguson would have hoped. There are signs of fatigue in his squad — no matter that he made nine changes to the weakened line-up that lost to Everton in the FA Cup semi-final on Sunday — and, while they started brightly, taking a deserved early lead through Wayne Rooney, they were stumbling towards the finish line until Michael Carrick, a substitute, made the points safe with eight minutes remaining.

Ferguson admitted as much afterwards, saying that the sloppy second-half performance had told him and his players they cannot take anything for granted over the weeks ahead. He conceded that his team, having played “some of the best football you will see all season” in the first half, had offered Portsmouth a route back into the game in the second period. It spoke volumes that Paul Hart, the Portsmouth manager, left Old Trafford frustrated, rather than relieved, by the final scoreline, having probably been worried that his team’s goal difference would take a battering after Rooney gave United a ninth-minute lead.

The malaise around Old Trafford is hard to explain, given that United have enjoyed uplifting victories over Aston Villa and Porto in the past few weeks, but it can be discerned in the fitful contributions of Cristiano Ronaldo and the unease in the stands about the intensifying debate over Carlos Tévez and Dimitar Berbatov. Only Rooney, of United’s attacking players, is in any kind of form at present, which may be one reason why Ferguson opted to keep both Tévez and Berbatov on the bench throughout last night.

It was telling that the former, who confirmed yesterday that he expects to leave the club at the end of the season after running out of patience at his lack of opportunities, was greeted with far greater enthusiasm when he went through the motions of a warm-up in the second half.

The match had been hyped, at least as far as the world’s most self-effacing footballer would allow, as a landmark night for Paul Scholes, who played with typical economy on his 600th appearance for the club he joined as a schoolboy. There was an eye-catching contribution from Ryan Giggs, who helped to set up the opening goal for Rooney, but it was another chastening evening for United’s third home-grown veteran, Gary Neville, who suffered a gash to his right foot in the early stages. Ferguson’s mood was not helped by the subsequent loss of John O’Shea, Neville’s replacement, also to a foot injury, inflicted by Nadir Belhadj’s ugly tackle early in the second half. The United manager announced through gritted teeth that both Neville and O’Shea will be out “for weeks”. The return of Wes Brown, back in full training after six months of almost constant injury problems, can hardly come quickly enough for Ferguson.

Neville lasted just long enough to see United claim the lead when Giggs ran on to a superb reverse pass from Anderson and crossed into the six-yard box, where Rooney, attacking the ball ahead of Sylvain Distin, slotted his shot past David James. It was an exquisite goal that seemed to signal the long-awaited return of a swagger to United’s play, with O’Shea seeing a header cleared off the line by Belhadj and Giggs twice squandering clear opportunities in a 25-minute purple patch that ensued.

By Ferguson’s reckoning, United should have been 5-0 up at the interval, which is probably like saying that Liverpool should have beaten Arsenal 13-4 the previous night. Portsmouth, after a highly unconvincing first 45 minutes, were fortunate to be only one goal behind, but United left their swagger in the dressing-room at half-time, offering Hart’s team an unexpected route back into the game.

United’s performance had suddenly gone flat and, as Portsmouth ventured forward for the first time, at first tentatively, hope grew among the opposition supporters. First of all they threatened through Glen Johnson, whose cross-shot was dealt with awkwardly by Edwin van der Sar, then Peter Crouch, volleying wide after Jermaine Pennant, on as substitute for the injured David Nugent, got behind the strangely unconvincing Patrice Evra.

Ferguson’s players were getting nervous — and so was he, to judge from his increasingly frenzied antics on the touchline — but the introduction of Carrick induced some much-needed composure.

With eight minutes remaining, Scholes threaded a perfect pass that allowed Carrick to run through and send a crisp low shot past James. Finally the supporters could relax and run through their repertoire of anti-Liverpool chants, but again there was none of their usual gusto. Something is not quite right here, but it is nothing, surely, that a large silver trophy or two will not put right over the coming weeks.

Manchester United (4-2-3-1): E van der Sar — G Neville (sub: J O’Shea, 12min; sub: Rafael Da Silva, 52), N Vidic, J Evans, P Evra — D Fletcher, P Scholes — C Ronaldo, Anderson (sub: M Carrick, 76), R Giggs — W Rooney. Substitutes not used: T Kuszczak, Nani, C Tévez, D Berbatov. Booked: Vidic.

Portsmouth (4-3-2-1): D James — G Johnson, S Campbell, S Distin, H Hreidarsson — R Hughes, S Davis, H Mullins — D Nugent (sub: J Pennant, 46), N Belhadj — P Crouch. Substitutes not used: A Begovic, N Pamarot, M Cranie, A Basinas, J Utaka, Kanu.

Referee: P Walton.