The Portuguese team suffered another blow when captain Lucho Gonzalez was forced off with a knee injury and had to be replaced by Mariano.

Alves posed danger again in the 40th minute, heading just wide after an excellent Raul Meireles free kick.

Porto continued to create chances but United, with England's Rio Ferdinand back in the side after missing the 2-2 first-leg draw through injury, defended resolutely.

Striker Lisandro Lopez (pictured) headed wide from a corner and substitute Ernesto Farias almost made an instant impact with a bicycle kick which flew over the United bar.