By Phil McNulty
Chief football writer at Old Trafford
Mariano stunned Old Trafford with his well-taken late equaliser for Porto
Manchester United's hold on the Champions League hangs by a thread after Porto left Old Trafford with a well-deserved draw - and two crucial away goals - on a disappointing night for Sir Alex Ferguson's side.
Carlos Tevez's strike five minutes from time looked to have given United an advantage they barely merited as they looked a pale shadow of a side on course to collect an historic haul of five trophies.
But as Old Trafford breathed a sigh of relief, Porto delivered what could turn out to be the telling blow when Mariano scrambled home a last-minute equaliser to rekindle dark memories of United's exit to the same team five years ago.
Cristian Rodriguez gave Porto the lead inside four minutes, his sharp finish coming as a result of the defensive uncertainty that plagued United from start to finish.
And even though Wayne Rooney swiftly levelled after he swooped on Bruno Alves' backpass, there was to be no escape for United.
If Federico Macheda's late winner against Aston Villa on Sunday was supposed to have rejuvenated United after Premier League defeats against Liverpool and Fulham, there was little evidence of their recovery at a subdued Old Trafford.
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.
Tevez (right) came off the bench to score United's second goal
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.
Porto posted the first warning inside two minutes when Lisandro Lopez's powerful shot was turned over the top by keeper Edwin van der Sar with United's defence exposed.
It was the shortest of reprieves and a sign of things to come as Porto were rewarded with an away goal after only four minutes that summed up everything about the difference between the two sides in the opening exchanges.
Cristiano Ronaldo conceded possession wastefully before a lapse of concentration from Jonny Evans allowed Rodriguez to cut in and curl a precise finish beyond Van der Sar to leave Old Trafford stunned into silence.
United's response was a Ronaldo header that was clawed away by keeper Helton - and they were allowed to draw level after 15 minutes courtesy of a monumental defensive miscalculation from Porto defender Alves.
Porto were in no danger as the defender decided to try and find keeper Helton with a back-pass, blissfully unaware that Rooney had anticipated the move. The interception was as inevitable as the finish, coolly lifted over Helton by the England striker once the keeper had committed himself.
This should have been the cue for United to resume normal service and emerge from their shaky start. The opposite was true and Porto were presented with a host of chances to regain the lead.
It's a difficult job now - Ferguson
Rodriquez shot straight at Van der Sar when well placed and Lopez was inches away from connecting with Lucho's pass at the far post with no United defender in the vicinity.
And Old Trafford vented its fury on John O'Shea when more hapless defending gave the appropriately named Hulk an opportunity which, fortunately for United, he wasted.
United improved as half-time approached and Paul Scholes almost gave them a half-time lead when he headed over from Patrice Evra's cross.
Ferguson's brisk march down the touchline as the half-time whistle sounded hinted at the dressing down United were surely going to receive - and they resumed with a predictable increase in energy levels after the break.
Eccentric keeper Helton kept United at bay as they finally exerted a measure of supremacy, turning Rooney's clever chip over the top and diving spectacularly to save Vidic's header from substitute Ryan Giggs's corner.
Rodriguez (right) gave Porto an early and deserved lead
Porto were still a danger on the break, and twice Van der Sar was called into action to save from Lopez and Raul Meireles as United received a reminder of the threat the visitors still posed.
Ferguson sent on Tevez, as demanded by the Old Trafford crowd, and he was rewarded when he profited from Rooney's superb flick to power a finish past Helton at the near post.
But as United looked to have staged yet another dramatic late show, there was one last twist when Lucho set up Mariano, who was left unmarked to bear Van der Sar at the far post.
United's players departed with only the cheers of Porto's fans to accompany them - and with an uphill task in Portugal next week to stay in the tournament.
Van der Sar, O'Shea, Vidic, Evans (Neville 72), Evra, Fletcher, Carrick, Scholes (Tevez 72), Ronaldo, Rooney, Park (Giggs 58).
Subs Not Used: Foster, Eckersley, Nani, Macheda.
Rooney 15, Tevez 85.
Helton, Sapunaru, Rolando, Bruno Alves, Cissokho, Lucho Gonzalez, Fernando, Raul Meireles (Costa 79), Lopez, Hulk, Rodriguez (Mariano Gonzalez 79), Costa (Madrid 90).
Subs Not Used: Nuno, Stepanov, Farias, Sektioui.
Bruno Alves, Helton.
Rodriguez 4, Mariano Gonzalez 89.
Konrad Plautz (Austria).
BBC Sport Player Rater man of the match:
Porto's Christian Rodriguez on 7.75 (on 90 minutes). Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.