Sir Alex Ferguson admitted Manchester United could not afford to gift FC Porto their late equaliser at Old Trafford on Tuesday night.
United head to Portugal next week knowing they must do something no other English side has managed in 11 previous attempts and defeat Porto on their own ground. Barring an even bigger score draw, the 2-2 scoreline means the Champions League holders must win or go out at the quarter-final stage.
"You have to think the last goal was one we could not afford to lose," said the United boss after Mariano's late strike.
"It was a terrible goal to give away when you are 2-1 up and seeing the game out. It is a difficult job now, not made any easier when they tell you no British team has won in Porto.
"But we only have ourselves to blame. We just have to win. We have been first at everything at this club. Let's hope we can be first again."
After their last-gasp heroics against Aston Villa on Sunday, United appeared to be caught off guard by Porto's flying start.
What was once the most watertight defence in Europe failed to come to terms with the threat Porto posed and the visitors could easily have scored more than the single first-half goal they did manage through Cristian Rodriguez.
The only reason Porto were not in front at the break was a disastrous mistake from Bruno Alves, who gifted Wayne Rooney a 15th-minute equaliser.
United got on top in the second-half and when Carlos Tevez struck at the near post, it seemed victory would be secured.
But Mariano stunned Old Trafford with a late goal that was no more than the impressive Portuguese side deserved.
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