The Old Trafford outfit came from behind but failed to hold on to their lead and must now become the first English team to win in Porto if they are to progress to the semi final stage of Europe's Premier football trophy.
Manchester Utd came into this Champions League quarter final tie on the back of a two successive league defeats ending with a fortunate last gasp victory over Aston Villa on Sunday.
Any thoughts that Champions League football would enable the Reds to shake off their lethargic league form were dispelled within three minutes of the start.
The warning signs were already there after only two minutes when the Brazilian Hulk was allowed time to hit a fierce shot which Edwin van der Sar punched behind.
A minute later and Porto, who have lost on ten of their eleven visits to England, took the lead.
Jonny Evans gave away possession too easily allowing Rodriguez time to drive the ball into the bottom corner.
United were looking shaky in defence and finding it difficult to string passes together, whilst Porto with the confidence of a goal, looked neat in possession and a constant threat with Hulk.
The Reds were creating little with only a Ronaldo header giving a moment of concern to the visitors.
But an awful defensive howler gave United the equaliser.
Bruno Alves inexplicably failed to look behind him and gifted a back pass to Wayne Rooney who coolly chipped Helton.
Instead of building on this fortunate goal it was the visitors and not the home side who took the initiative.
Without the commanding presence of Rio Ferdinand at the back Porto's quick, incisive breaks were causing uncertainty in United's rearguard, whilst too many careless passes meant the Porto defence was rarely tested.
Darren Fletcher had a shot charged down but that was about as good as it got for the defending Champions of Europe.
The Porto fans were in fine voice compared to their subdued Old Trafford counterparts.
Sir Alex Ferguson's half time talk certainly had some effect as United began the second half with more urgency and pace.
United were looking more purposeful as Porto began to defend deeper and an audacious Rooney chip from distance had the keeper desperately palming over - from the resulting corner Nemanja Vidic went close with a header.
Van der Sar dealt with a deflected shot from Lisandro then saved a long range effort from Cissokho as Porto begin to look dangerous on the break.
But, with five minutes remaining substitute Carlos Tevez got on the end of a Rooney flick-on to hammer home and seemingly give United a lifeline for the second leg.
However, the home side's defensive frailties were exposed within a couple of minutes when substitute Mariano stuns the 75,000 crowd with a second away goal for the Portuguese outfit, stealing in unmarked at the far post.
"Yes, the result was about right.
"They were the better side in the first half, but in the second half we improved our game and had a lot of possession and getting the goal when we did we should have held on. It was a bad goal to lose.
"It might have been fatigue after two games in two days or more likely a loss of concentration.
"Porto played as I expected so that was no surprise but it's certainly going to be a hard return game over there."