Wayne Rooney answered Manchester United's FA Cup SOS at Old Trafford as the Red Devils overcame two horrendous moments of ill-fortune to book a place in the fifth round at Portsmouth's expense.

Rooney was only introduced on the hour as United struggled to make the decisive breakthrough against their Barclays Premiership rivals.

But, having broken a seven-match drought at Arsenal last weekend, the 21-year-old followed up with a late double, the second a sensational chipped effort from 20 yards.

In contrast to his staggering second effort, Rooney's opener - his 10th of the campaign - was a tap-in, provided by Ryan Giggs.

It helped relieve the tension of a game which seemed destined to finish deadlocked, probably saving referee Mike Riley a hairdryer blast from Sir Alex Ferguson in the process.

On this ground almost two years to the day ago, while he was still at Tottenham, Pedro Mendes was on the wrong end of an abysmal call when none of the officials managed to spot Roy Carroll dropping the midfielder's shot a yard over his own line.

At the same end, just 12 minutes into an intriguing contest, Mendes finally gained some recompense, hooking a Nemanja Vidic header parried into his path by David James away from danger - despite the ball clearly crossing the line.

If that was not bad enough for the hosts, they also saw a perfectly good Henrik Larsson volley incorrectly ruled out for offside after half-time, a moment which must have left Ferguson wondering how many ladders he had walked under recently.

In the end though, Rooney's arrival ensured there was no need for recrimination, even if Mendes' late deflected effort meant United were not able to book their spot in the last 16 without some anxiety.

Although it was half a century since Portsmouth last won at Old Trafford, they started in enterprising fashion and had a half-hearted penalty appeal turned down when returning skipper Gary O'Neil fell under Tomasz Kuszczak's sliding challenge just four minutes into the game.

He did not know it at the time, but it was not to be the biggest 'non-decision' Riley would make, nor the absolute irony of it when the most controversial ruling came.

Clearly, the old adage 'what comes around, goes around' was lost on Ferguson, who raced to the edge of his technical area to berate Riley for the error, the nearside assistant who made the call conveniently placed for an earful from the United boss, who had the advantage of watching TV replays.

Stung by the injustice, the hosts shook off the lethargy that bedevilled their earlier efforts and went for the jugular.

Park Ji-Sung was particularly prominent, as was Larsson.

A well-timed Sol Campbell tackle prevented Park from bursting clear and the South Korean only just failed to meet Patrice Evra's low cross with a diving header.

Larsson's hopes of steering home Ryan Giggs' cross were snuffed out by Lauren and, as the opening period drew to a close, United's danger pair combined to create an opportunity Park blasted over.

Their best chance though, other than Vidic's header, was a virtual replica move as Giggs swung a corner onto Michael Carrick's head, with David James magnificently tipping the goalbound effort over.

With Rooney on the bench and Cristiano Ronaldo not even in Ferguson's squad after spending a short break in Dubai, United's attack lacked raw pace and, with Campbell imperious, Portsmouth's defences held firm with a bit to spare.

The visitors gained enough confidence from their first-half endeavours to push forward a bit more after the interval.

Linvoy Primus guided a back-header onto the roof of Kuszczak's net, then former United Cup winner Andy Cole disappointingly prodded Mendes' cross straight at Rio Ferdinand when well-placed to do much better.

If there was a nagging feeling this was not going to be United's night, it was only heightened by a series of events building up to the hour mark.

Firstly, to his total disbelief, Larsson was ruled offside, then James somehow palmed away a Paul Scholes rocket before Ferdinand failed to divert a twice-deflected Giggs cross into the net from barely six yards.

The introduction of Rooney for Solskjaer quickly followed and the youngster's arrival instantly offered Giggs more room to work his magic.

It was no surprise the Welshman should tee up Rooney for the opener either, although Larsson deserves a special mention for the calm lay-off which ensured Giggs would leave his young team-mate with no harder task than an empty net to find.

The best was yet to come though as Rooney collected Gary Neville's pass 20 yards out, before beating James with an awesome chip.

Kanu breathed new life into the contest three minutes from time when he deflected in a Mendes shot - but United were not to be denied.

Teams

Man Utd: Kuszczak, Neville, Ferdinand, Vidic, Evra, Park, Scholes, Carrick, Giggs (Fletcher 85), Larsson, Solskjaer (Rooney 60).

Subs Not Used: Van der Sar, O'Shea, Silvestre.

Goals: Rooney 77, 83.

Portsmouth: James, Johnson, Campbell, Primus, Hughes (Douala 82), Lauren, O'Neil, Pedro Mendes, Taylor, Kranjcar (Mwaruwari 80), Cole (Kanu 62).

Subs Not Used: Ashdown, Thompson.

Booked: Hughes.

Goals: Pedro Mendes 87.

Att: 71,137

Ref: M Riley (W Yorkshire).