United will have to make up for the disappointing first-leg 2-2 draw at Old Trafford last week, which means they must score at Estadio Dragao.

But Alex Ferguson's team, who uncommonly leaked in many goals of late, will have a tough task against a Porto side unbeaten in 20 games.

The Red Devils are boosted by the return of key center-half Rio Ferdinand, who missed the Old Trafford game with a groin injury.

Dimitar Berbatov also returns from injury and could partner Wayne Rooney upfront.

Apart from Jorge Fucile (ankle), Porto have no real injury concerns; but their manager Jesualdo Ferreira will have to watch the game from the stands.

Ferreira was handed a touchline ban by UEFA for his inappropriate gesture towards the referee when Porto had a goal disallowed against Atletico Madrid in the previous round in February.

Gunning for Glory

At Emirates Stadium, Arsenal are firm favorites to book their semi-final place when they host an injury-hit Villarreal side, who only managed a 1-1 draw at home last week.

Arsenal, who thrashed Wigan 4-1 at the weekend, seemed to have regained their goalscoring momentum with the recent return of Cesc Fabregas and Emmanuel Adebayor from injury.

On the contrary, Villarreal suffered back-to-back defeats in the Primera Liga at the hands of Almeria and Malaga respectively.

The Gunners have a host of injury problems at the back, with William Gallas, Gael Clichy and Johan Djourou all ruled out with injuries.

In addition to that, first-choice keeper Manuel Almunia is out and Lukasz Fabianski will deputize.

Nonetheless, Villarreal's injury list is in no shortage of big names as the Spaniards miss influential midfield duo Santi Cazorla and Marcos Senna.

Cazorla is out for the season with a broken leg, while skipper Senna pulled a hamstring at the weekend's defeat to Malaga.