United gave away possession at will and displayed the sort of alarming defensive vulnerability that was first exposed in the home defeat against Liverpool.
Throw in a disjointed midfield and an attacking effort that relied almost solely on the inspirational Rooney, and all the ingredients were in place for the visitors to produce a shock.
Porto, in contrast, were sharp on the ball and bright in attack - encouraged by the inability of their opponents to apply any concerted pressure on their defence.
United were able to recall Rooney and Nemanja Vidic after their domestic suspensions - but Rio Ferdinand was still absent with a groin injury.
Slack at the back, careless in possession and lacking any spark of invention, their only comfort was that they reached the interval on level terms after being outplayed and outsmarted by Porto.
BBC Website