Manchester United face a tense battle to retain their place in this season's Champions League after failing to hold on to the late advantage provided by Carlos Tevez.
Although United were outplayed in the first half, Wayne Rooney's fortuitous 14th goal of the season had ensured the Red Devils were level heading into the final stages.
When Tevez turned home Rooney's back-flick five minutes from time, Sir Alex Ferguson's men must have thought they had come through a very difficult evening.
But not for the first time, they lost concentration in defence and with just a minute left, Mariano found himself totally unmarked as he finished off a cross from the excellent Lisandro.
Ferguson had played down the belief that his side had it easy. The fact their Portuguese opponents had finished above Arsenal in the group stages and dumped out a highly-rated Atletico Madrid outfit in the last round seemed to get overlooked in the assessment.
Yet even before they scored, Porto had served notice that their speed and slick passing game was going to be difficult to contain.
Lisandro drew an excellent save out of Edwin van der Sar after leaving Evans looking distinctly flat-footed.
The young Northern Ireland defender has drawn warm praise for the way he has stood in for both Rio Ferdinand and the returning Nemanja Vidic this season.
But against Aston Villa last Sunday came the first sign that the strain is starting to tell on the 21-year-old.
And after one escape, Porto were in no mood to offer another as a slack pass from Cristiano Ronaldo put United under pressure, just as it did on Sunday.
Lisandro swung over a cross that Evans should have dealt with comfortably. Instead, he prodded the ball straight to Rodriguez, who took a stride forward before calmly curling a shot into the bottom corner.
Lisandro tested Van der Sar again shortly afterwards as United again found themselves indebted to the keeper who, at 38, was preventing a total collapse.
It was the same kind of mauling United inflicted on Inter Milan at the San Siro during the first leg of their last 16 meeting in February.
Ferguson left Milan that night wondering how his side had not come away with a victory. Porto counterpart Jesualdo Ferreira must have been feeling exactly the same at half-time given United had managed to drag themselves level.
There was certainly no need for an inquest as to how the home side got their goal. There was not a hole big enough for Alves to leap into as he contemplated his catastrophic mistake.
Alves presumably thought he was doing exactly the right thing when he turned Aly Cissokho's square pass back to Helton.
Unfortunately for the Portugal international, he had not bothered to look and therefore spot Rooney in a position that required only the finish, which the England striker delivered with nerveless authority.
It was another 20 minutes before United created an opportunity by themselves, which Darren Fletcher cannoned into Cissokho.
The intervening period had been spent chasing shadows as Lisandro, Hulk and Rodriguez made a mockery of Porto's status as competition outsiders.
Ferguson used the interval wisely judging by the way his team began the second period, showing a bit more of the finesse Old Trafford is used to.
Still, it took a moment of inspiration from Rooney to threaten the visitors' goal as Helton touched the striker's sublime chip over.
Ryan Giggs' corner brought a more routine opportunity as Vidic climbed highest only for Helton to prove the equal of a powerful header.
For all United had gained a solid foothold in the contest, Porto were still an electrifying attacking force. Lisandro and Cissokho both brought decent saves out of Van der Sar before Carlos Tevez, chasing back shortly after being introduced as a substitute, seemed to chop Hulk down in the area.
It did not come anywhere near the good fortune Rooney had profited from earlier but as a whole, Porto might have started to wonder whether the crucial element of luck would be missing from their quest to reach the last four.
When Tevez turned home Rooney's sublime back-flick five minutes from time, it seemed those fears had turned to reality.
But, as the celebrations rang round the stadium, Porto powered up to the other end where Mariano did his own supersub impression, drifting into space at the far post to finish off Lisandro's cross.Reuse content