In a week when Cristiano Ronaldo has been hailed by Alex Ferguson as a possible equal to Pele and Maradona - and signed a new £3m (€4.4m) contract - Rooney proved the Portugal winger is not the only brilliant youngster at Old Trafford.

The 21-year-old lashed United into an early lead against Watford, then teed up Ronaldo to put them back in front following Hameur Bouazza's leveller.

With a booking for a feisty challenge on Tommy Smith as well, it was vintage Rooney and his tap-in put United on easy street long before Kieran Richardson finished off the scoring with a neat effort.

Yet amid the celebrations of an eighth final appearance under Ferguson, and the prospect of a mouth-watering clash with Chelsea, the concerns in the United camp were obvious following the loss of Rio Ferdinand to a groin injury.

With John O'Shea missing, Nemanja Vidic and Mikael Silvestre sidelined with long-term problems and Gary Neville at least a week away from a return to action from his ankle problem, Ferguson's defensive options have been ravaged to the extent Darren Fletcher was forced into the right-back role, with rookie Youth Cup star Craig Cathcart the only alternative.

At such a crucial stage of their treble bid, and with so many opponents to face of far superior quality than Championship-bound Watford, United are facing the biggest test of their season.

Sheffield United, fresh from their hammering of West Ham, meet United on Tuesday, although Ferguson will at least allow his players the luxury of some celebration before addressing the Blades' visit.

For, while never hitting the heights of their seven-goal rout of Roma, the Red Devils still managed a thrill or two, and in Rooney had a stand-out star who looks set to be a major influence on United's bid for honours over the coming weeks.

Having witnessed United's awesome strength first hand on Tuesday, the last thing Aidy Boothroyd would have wanted was for his side to concede as early as the sixth minute.

The spellbinding movement of Rooney, Ronaldo and Ryan Giggs had already caused Watford a few headaches before they were opened up in startlingly simple fashion.

Gabriel Heinze started the move with an inside pass to Michael Carrick. The former Tottenham man quickly fed Rooney, who stepped inside Adrian Mariappa and promptly smashed the ball into the roof of Richard Lee's net.

Fists raised in triumph, Ferguson emerged from his dug-out to celebrate, the Red Devils fans roared and Villa Park awaited a slaughter.

The intense pressure duly came but Watford gamely clung on and received an unexpected reward when Bouazza levelled.

United's defence appeared to lose concentration during a lengthy stoppage while Edwin van der Sar had treatment for a bang on his nose.

Jay DeMerit helped back a long Gavin Mahon throw back into the box and Bouazza got a firm strike on a volley which bounced in off the crossbar after flicking off Heinze.

Had Watford remained on terms for any length of time, particularly given Ferdinand's early departure, they would have scented a shock.

Instead, their equaliser merely ignited their opponents - and Rooney in particular - to greater effort.

The England striker has had his critics this season but his determination to shove United in front again was immense.

Charging to the touchline, Rooney first won possession with a robust aerial challenge on Jordan Stewart. He then totally outpaced the Watford full-back as he strode onto Smith's return pass before delaying his cross long enough for Ronaldo to steal between two defenders and tap home one of the easier goals of the 21 he has scored this term.

Yet again, United were dominant, some of their play mesmerising.

But their enthusiasm was cut short by Ferdinand's glum trudge to the touchline, accepting the futility of carrying on in such pain.

His exit triggered a defensive reshuffle which saw Fletcher fill in at right-back, with Heinze alongside Wes Brown in the centre.

The uncertainty within United's defence was obvious immediately after the re-start as Bouazza volleyed a good chance wide after another Mahon through created mayhem.

Van der Sar was then required to palm a Bouazza cross-shot away from his own bar and the sight of Ferguson exchanging angry words with Giggs was a sure sign of tension in the United camp.

Thankfully for Ferguson, his attack remains in tact and after providing Rooney with a tap-in, Smith fed Richardson, who completed a win which looked much more comfortable than it was.