FA Cup/ Manchester Utd 2 Portsmouth 1: The fourth official's board summoned Ryan Giggs from the fray five minutes from normal time and, as United's stalwart strode from the turf, he made a beeline for the man who had chiselled this victory.

There followed a pat of thanks and a brief shake of the hands, all of which were barely acknowledged amid the crescendo of applause for the retreating winger. But if ever one subtle show of appreciation could sum up Wayne Rooney's value to the club, this was it.

Rooney is destined to illuminate occasions such as this. Portsmouth, awkward and rugged, had been squeezing the belief from the home side when, on the hour mark, Alex Ferguson turned to the England striker in hope of his opening the door to progress.

What ensued was a master class in what the 21-year-old already offers as well as an indication of what he will provide more regularly in the future.

"Sometimes people expect too much of him," said Rio Ferdinand afterwards. "People shouldn't forget he is still a young player on a learning curve."

He is already a phenomenon; the first touch and lob over an aghast David James on Saturday was a reminder of his talent. Any frustration with the forward centres on how he scores only in fits and starts. His 11 United goals this season have been plundered in six of his 30 club appearances.

Otherwise he has been left to grumble through lengthy barren spells. The first real slump of a glittering career has been posited but the consistency he craves will surely come in time; for the moment there is an obvious comparison to be made in the flashes of brilliance he offers.

"That second goal was one of imagination and audacity," said Ferguson. "It was the kind of thing that Eric Cantona used to do and Wayne is the kind of boy who has the courage to try it."

Had the home side's latest kit offered a collar, Rooney's would no doubt have been flicked up as he stood, arms aloft even before his lob kissed the underside of the crossbar before ruffling the net, as if waiting for the arena to catch up and erupt in recognition.

Like Cantona, Rooney is as much a creator as a scorer. But, while Le Roi arrived at Old Trafford bolstered by years of experience, Rooney is still acquiring comparable knowledge.

Even so, he sees passes where others spy only dead-ends, searches out space in the most smothering of defences. Sol Campbell and Linvoy Primus were outstanding here but still found the substitute elusive and, when he allies that with sharpness in front of goal, half-hour cameos such as this can erode the most stubborn of opponents.

"During this spell, when people have been questioning his goalscoring, Wayne's work-rate and team ethic never wavered at all," added Ferdinand. "He's a joy to play with and watch."

Portsmouth would not share that delight, even if they had to acknowledge the striker's genius.

The visitors were denied a potential penalty early on, Gary O'Neil dismayed at the non-award after tumbling in a challenge with Tomasz Kuszczak, and survived Nemanja Vidic and Henrik Larsson scoring legitimate-looking goals courtesy of oversights by an assistant referee.

Vidic's header appeared to cross the line before being cleared while Larsson's superb volleyed finish was ruled offside. When Andrew Cole failed to avail of a good chance early in the second period Portsmouth's chance was effectively gone.

Rooney's introduction yielded reward in 17 minutes, O'Neil surrendering possession for Michael Carrick and then Larsson to supply Giggs, with the Welshman's cross tapped into the unguarded net by the substitute.

"In a way I am more pleased with that one than the second as I need to score easier ones," said Rooney, whose sublime chip - anything but simple - took his tally to three goals in two games. Nwankwo Kanu, deflecting in Pedro Mendes's drive, registered consolation near the end.

Portsmouth, having proved this awkward, can return to their pursuit of European qualification via the Premiership with real self-belief. For United the prospects appear brighter still.

The league defeat by Arsenal has been partially exorcised and their three-pronged assault on honours maintained. Ferguson is suitably encouraged and added: "If Wayne gets on a scoring run like he did last year," he said, "it'll make some difference to us."

• Guardian Service