The 2004 Champions League winners were certainly no pushovers as they had shown by beating Atletico Madrid in the last round.
Porto started the game very brightly and Givanildo Vieira Hulk let fly with an early rasping 25-yard shot that Edwin van der Sar was forced to tip over the bar.
The visitors took a shock lead in the fourth minute when poor play by Cristiano Ronaldo and Jonny Evans allowed Cristian Rodriguez to fire a low shot into the corner of the net.
United took a long time to settle and they nearly fell two goals behind in the seventh minute when van der Sar was forced to make a smart save to deny the lively Lisandro Lopez.
In the 14th minute, United created their first opening when Helton was forced to keep Ronaldo's header out, and 60 seconds later an appalling error allowed Sir Alex Ferguson's side to equalise.
Bruno Alves didn't look up as he played a disastrous back-pass straight to Wayne Rooney who calmly lifted the ball over Helton and into the net.
The goal didn't deter Porto though and, in the 19th minute, Rodriguez should have restored their lead when he contrived to head the ball straight at van der Sar.
On the half-hour mark Hulk clipped a shot inches wide after John O'Shea's error had let the Brazilian in.
United finally improved just before the break and Darren Fletcher's shot was deflected away for a corner after a great pass by Paul Scholes and cushioned header by Ronaldo had created the opening.
Scholes then headed Patrice Evra's cross over the bar but Porto still seemed capable of ripping a very disjointed United defence to shreds.
United improved in the second period and, on the hour mark, Rooney was close to giving Ferguson's side the lead when he almost caught Helton out, forcing the keeper to hurry back to his line and tip the ball over the bar.
Nemanja Vidic then powered in a header that was clawed away by the Porto keeper as the visitors struggled to quell United's attacks.
Carlos Tevez was fortunate not to concede a penalty when he appeared to bring Hulk down only for the referee to wave play on, and it was the Argentinian who gave United the lead with five minutes left when he benefited from Rooney's precise flick.
However, with only one minute of normal time left, a cross found its way through to Gonzalez who was on hand to fire his shot past van der Sar to set up an intriguing second leg in Portugal next Wednesday.